I had 4 out at once recently two at 3 days, one at 6 days and one at 8 days I think it was.

I play a 4-1-1-2-2 formation, I have two players for each position but as it happened both my MC's was injured at the same time, thankfully I had stocked up on red heal packs.

Annoying but as some have said part and parcel of the game.