Er... I play on Android. Don't have Facebook (doesn't want to have that) so can't play there. On phone, I tap on my player, then keep tapping on the injury until it says fit. If the injury is for one day (when I don't use reds) then 5 packs heals him. 2 days then 5 packs reduce recovery time by a day, 9 heals. 3 days injury then 5 reduce 1 day, 9 reduces 2 days, 12 heals. To sum it up:
Reds—Days Reduced
5—1
9—2
12—3
14—4
15—Heals

If the healing time matches with the med pack reduction time then the player will be healed. If an injury is for 3 days, then you can spend at the most 12 reds at a time and no more, which'll heal him.