3. Juan Mata:

The 3030-year-old Mata has been with United since 2014, but found it somewhat difficult to become a regular starter for them. He is a skillful player with good passing and shooting abilities, but has succeeded in scoring only 44 goals in 209 matches for United so far.
He has already been linked with other English clubs like Arsenal, and made 23 appearances for United this season so far. Moreover, most of those appearances have come as a substitute only. Players like Ander Herrera, Nemanja Matic and Paul Pogba have been superb under Solskjaer, and lent solidity to the United midfield.
United are already having talks with attacking midfielders like Antoine Griezmann for the next season and Mata should find it increasingly difficult to find a place in the starting eleven. Therefore, it might be time for him to reconsider his options, and make a switch to some other club. He could sign for any other English club or even return to Spain to play in the LaLiga. Either way, this could very well be his last season with United.
In any case, Mata's contract with Manchester United ends this summer, so he might well be allowed to leave on a free transfer.