As the summer transfer window approaches, the future of Manchester United forward Cristiano Ronaldo is in doubt. Several clubs are interested in acquiring his services, but the Red Devils have so far refused any bid. Meanwhile, the Portugal international has been keeping himself fit in Lisbon. What is he up to in Lisbon? Read on to discover more. In the meantime, here are some things we know about Cristiano Ronaldo.Ronaldo's future at Manchester United is up in the airCristiano Ronaldo's future at Manchester United is in doubt, but a recent report claimed he has not decided whether to stay at the club for next season. The Portuguese forward has not been taking many chances in front of goal lately and missed out on the Manchester derby against Manchester City. With his future uncertain, he may decide to leave the club in search of better opportunities in Europe. Ronaldo's absence from United this season would mean an increased demand for a younger goalscorer.Despite the upheaval in his relationship with the club, United have not ruled out the prospect of selling the player in a blockbuster deal. As such, the club are trying to find a solution that will please both sides. Ten Hag has stated that he wants to move elsewhere but is also pursuing his own path. Until then, Ronaldo is likely to stay and fight for a spot at United.The season has gone from bad to worse since Ronaldo's return to the club. While the team struggled under Solskjaer last year, Ronaldo's return has disrupted their season, blunting their speedy attack. Young stars have been relegated to the periphery while the club's formula for success was diluted by his need to accommodate Ronaldo. The Portuguese international has also struggled to press in the modern game and his dramatic frustration sets a negative tone for the season.Several clubs are interested in signing himVarious clubs are interested in signing Ronaldo, but few can match the Brazilian's ambition and high wages. Chelsea are heavy favourites, but their manager Thomas Tuchel is adamant that he will not sign Ronaldo. In the meantime, Chelsea are close to signing Raheem Sterling. But the Blues have decided not to sign the winger after meeting with Chelsea agent Todd Boehly.Bayern Munich and Paris Saint-Germain have also been linked with a move for the Brazilian forward. Despite Real Madrid's failure to make a move for Ronaldo, Bayern Munich and AS Roma have also shown an interest in signing the Portuguese striker. The latter is keen to reunite with Jose Mourinho and play Champions League football. But Manchester United are keen to keep Ronaldo and he could end up playing Europa League football for the first time in two decades.Manchester United and Juventus were the first to make an approach for Ronaldo. Real Madrid were unwilling to pay the high asking price, but the latter refused. Ronaldo had been at Manchester United for six years, winning three Premier League titles and the Champions League. But Manchester United's bid was not up to scratch, so the Italian club opted to wait for another year. After all, it would be a mistake to sign Ronaldo to a team that has no chance of winning a Champions League title.United have so far rejected any moveSince the player announced his intention to leave Manchester United in the summer, few clubs have expressed an interest in him, with only one concrete bid. Manchester United have turned down offers from Chelsea, Bayern Munich and Real Madrid, and Atletico were the only club to express interest. However, Atletico would have to sell some players to fund the move. Despite this, United have so far refused Ronaldo's request to return to Manchester for talks.It's unclear whether Ronaldo will train this week. However, the player is expected to meet Ten Hag face to face this week, and there are reports that the pair will try to resolve their stand-off with United. Earlier this summer, Barcelona, Bayern Munich, and PSG were among the clubs monitoring the situation. With no real alternative in sight, it's hardly surprising that United have so far rejected Ronaldo's request to return to Manchester for talks.While the return to Manchester is seen as a positive sign for the club, it's important to remember that it is still early in the process and United's top executive, Richard Arnold, has not made any decision on the matter. The player's agent, Jorge Mendes, has contacted the player's representatives in an attempt to secure a deal for the player. The club have said that Ronaldo is not for sale and will remain part of their plans for the season ahead.Ronaldo has been keeping himself fit in LisbonIn addition to his lavish lifestyle, Cristiano Ronaldo has been keeping himself fit in Lisbon. The Juventus forward purchased a PS6million penthouse there in July 2018 and has been keeping himself fit. It features a gym and swimming pool. The apartment also has three large bedrooms and a spa. It is also known as the 'Ronaldo mansion', and it has all the luxuries you could ever want, including a swimming pool.Although he was due to return to Manchester United for pre-season training on July 4, the Portugal trip has kept him fit. During his stay in Lisbon, he has participated in drills alongside Sheffield United's senior team. Nevertheless, the Portuguese international may not be ready to join the rest of the squad immediately. However, his return to training is seen as a positive sign. He is likely to stay in the United camp for another season despite a rumour of a move away.While he does spend a significant amount of time training and playing football, he also eats a simple diet of fish and vegetables. He limits his meat intake and opts for fresh fish and vegetables. In Lisbon, Ronaldo enjoys the local dish Bacalao a la Brasa, which consists of cod, potatoes, onions, and scrambled eggs. Ronaldo makes sure to eat six small meals throughout the day, and he tries to keep his diet as simple as possible.He is not lazyCristiano Ronaldo has made it clear twice that he will not join Manchester City if the current transfer saga continues. The Portuguese international has said he wants to stay with Manchester United after being linked with a move to Chelsea. However, the Blues are not interested in spending too much on their star man, so the club has made it clear they will not give in to Ronaldo's demands.The five-time Balon d'Or winner has been training with a private trainer in Portugal, but he may not be ready to take part in full training with the rest of the team. United's general manager Erik ten Hag has remained mum on Ronaldo's future, but he has hinted that the Portuguese international will stay at the club at least another season. The 29-year-old has 12 months left on his current deal, with an option to extend it by another year.According to sources in the club, the Portuguese forward will be back in Manchester on Monday, and he is expected to meet with new manager Erik ten Hag on Tuesday. After missing the first three weeks of pre-season due to a family matter, he is set to meet ten Hag for talks. United have been relying on Ronaldo to stay at Old Trafford next season, which is why they have switched their transfer focus from a striker to a more versatile forward. They have already missed out on Darwin Nunez, but have since signed Anthony Martial and Victor Lindelof.He can reinvent himself at 37The question is, can Ronaldo reinvent himself at 37? While most professional footballers do change as they age, few world-class superstars stay active into their late 30s. A few of them, such as Lionel Messi and Diego Forlan, have managed to reinvent themselves in their later years. For the rest of us, the question is, can Ronaldo reinvent himself at 37? And if he can, what role should he play in his club's rebuilding process?It is possible that Ronaldo can reinvent himself at 37. As a player who has been playing at the highest level since he was 18, he has a long and illustrious career. His recent success with Juventus has made him the highest goalscorer in the history of football. Earlier, he had a relationship with fitness coach Giovanni Mauri, who was also a former coach of Carlos Ancelotti at several clubs. Then, he played with AC Milan after suffering a devastating ACL tear.Manchester United and Ronaldo have a good history together. Both have been Champions League winners, and Ronaldo was instrumental in their successes. In addition to winning the Champions League and FA Cup, he also helped Manchester United win three Premier League titles. However, he left Manchester United in 2009 and joined Real Madrid for PS80million. While he enjoyed greater success in Spain, Ronaldo returned to Old Trafford last summer. Clegg worked with Ronaldo during his first spell at Manchester United and has shared his insight into his development.He is not fitAccording to reports, Cristiano Ronaldo has not yet returned to training with Manchester United and is not expected back until Tuesday. Ronaldo missed the United pre-season tour in Thailand and Australia on Monday due to family issues. United also agreed to allow him to delay his return to training. The Portuguese star has said that he is keen to play Champions League football after a disappointing season last season. The club is reportedly interested in bringing Ronaldo back on loan to improve their record in Europe's premier competition.However, there is still a chance that the two parties will reach an agreement before the end of the summer transfer window. Manchester United have remained staunch in their stance that Ronaldo is not for sale and have agreed to a timescale for talks to conclude before the transfer window closes on August 31. It is therefore expected that talks will take place over the next few days. This will ensure that speculation does not drag on until the end of the transfer window.While the two-year contract between Ronaldo and United expires in June 2018, a one-year extension is a likely outcome. The Portuguese international is now a highly rated player and commands half a million pounds a week, according to Sky Sports. However, the transfer deadline has been delayed since August, when Atletico pulled out of the race for the Portuguese star. In addition, Ronaldo has reportedly become the centre of attention at Old Trafford and is not a fit player to return for talks.