Before you give up on them, criticize their form in the player interaction screen, and then give somebody else a chance on your starting team for a few games. If they are playing badly but you continue to play them in every game and never let them know they're aren't playing well enough, they won't have any incentive to improve.
If you want a short term option, like somebody mentioned already, loan M Diarra from Real Madrid, he barely costs anything (something like 400k initial fee, 50% wages).