Fastest way is use green packs, spend 2~3 a day and the player should be back to shape in less than a week.

If that is too costly and you want to keep your player, let him play 20~60minutes in match (by subs, in or out) and never whole time until his form changed to 7 or more. This may weaken your team in a few games.

The slowest but still effective way, bench him while he have max morale. Keep him benched until his morale drop to orange or red. After that just use him as sub. If done well he will have good form and his morale will increase after playing a match. This can take a week or two.