patience, multi account, young player training. i have huge experience in helping player come back to his top form. but a star is very long to go and only World class player or player with good high end 3-4 8-9 or young player fast trainer can be expected to keep in your team. it's only the last solution if you really want to keep the player.
before waiting you should play the player out of his position and sub him in some friendly matches. one he can get 7 point for 2 matches consecutive and 0 matches with 5 rating in his last 5 matches he could gain his form again.