#1 Juan Mata
Another player whose age is starting to show is Juan Mata. He's set to turn 31 years old in April, which is around the time when managers need to question whether the player has it in them anymore.
Still, Mata has featured in 19 games for the Red Devils this season, scoring two goals and assisting three, but it's certainly nowhere near the level he was at a couple of seasons ago. His decline is obvious and it's not fair to expect him to be as good as he once was.
With interest shown by the likes of Juventus and AC Milan, Juan Mata could be set for the Serie A in the summer, which would be better for his style of play now. The division is relaxed and not as tough as the Premier League, so whilst any form of a challenge might be eradicated over there, it would still serve as a reasonably high standard of football for him to play at, without being made to look a fool.
Mata has a market value of around £20m, so if Manchester United can sell him this summer, they'll make a decent amount -- far more than they will if they let him stick around for any longer.