The limitations of Cristiano Ronaldo's physique are beginning to become apparent, especially after Manchester United's elimination from the 2021-22 FA Cup Fourth Round when Middlesbrough battled back from a goal down in Manchester to win an incredible 16-shot spot shootout. The Portuguese veteran, who turned 37 on February 5, failed to convert a penalty kick in the first half.

His romantic return at Old Trafford, where he spent six magical years filled with vast goals and accolades from 2003 to 2009, was anticipated as the move that would re-establish the club as title contenders, but the squad has mysteriously gone off the title radar. He opted to make a sensational comeback to England, where toughness is the most important characteristic, but his body just can't keep up.

In spite of the number of goals he has scored this season, Ronaldo is no longer the difference-maker like he used to be at Real Madrid and Juventus, and he is unable to assist the squad in the ways many imagined. Problems are just piling up for the Red Devils, and now, CR7's future has been put into question. Thus, the Premier League giants have already discovered his possible long-term replacement.

Manchester United want Alexander Isak to fill in Cristiano Ronaldo's shoes

Alexander Isak of Real Sociedad. (Juan Manuel Serrano Arce/Getty Images)

Alexander Isak of Real Sociedad. (Juan Manuel Serrano Arce/Getty Images)

The 22-year-old centre-forward Alexander Isak of Real Sociedad has apparently been identified as a potential Cristiano Ronaldo successor. The Portugal captain has a contract with Manchester United until summer 2023, but there have been rumors that he may go sooner than expected due to the club's poor season.

Although United have the opportunity to trigger a 12-month extension for Ronaldo, the five-time Ballon d'Or winner is obviously preparing for life without him. As a result, Ralf Rangnick's side are reportedly contemplating a bid for Sociedad attacker Isak ahead of Ronaldo's potential summer exit, English publication The Mirror says.

When being spotted in London on deadline day, the Sweden international sparked talk about a move to Arsenal, but he stayed in La Liga after the window closed. The White and Blues apparently refused to initiate negotiations with other teams until they were offered a sum that was equal to the Sweden international's release clause.