Afaik the only way to restore the performance of a player is to use green packs and train some. If you let him play 2 matchs a day, spent him at least one green everyday and his form would turn back to normal rather quickly.

Obviously there is a hidden value that increases the training speed if a player has played a match. Once it reachs the upper limit the performace in match drops horribly. The reverse is also true - a player in reserve for half of a season, but trained regularly, would likely perform exceptionally well for quite a few matchs, even if he is only 2* or something.