Does anyone else have an injury prone player?
I don't think that's the problem. It just because if you heal a player who has a 5 days injury, then it's very possible to have an injury in the next 1-2 days (bad health/physical situation is just hidden).
That's not a rule but something common. You can get away without your player get injured again for the next of the season.
Why is that ? Pure luck

* the only thing I 'm doing is that after healing my player, leave him out of games (and training too) for an extra day - but it's not working every time
** and another annoying thing is that your good player, after injured-healing-injured-healing ...maybe starts playing bad