1) Next season is another season so maybe he will improve.
This happening sometimes, as the opposite.
2) If you 're playing with AML-AMC-AMR-ST, usually the AMC is underperforming (compare to wingers) but still contributes to the team (at least more than a 2nd ST even if it's not so clear).
3) The MCs have mediocre performance and stats but still a MC with high quality contributes to team's domination and performance.
Also this player with 3 good positions is a useful coaching tool.