Injurys are part of the game, sometimes a bit to mutch for my taste, but If you have a 22 players squad, you do not need to waist reds.
My maine Team have now 100 red packs, in the last 2 seasons i only used them once. I just let the player heal by himself and another plays on his place, If i do not have a player to cover His position, i just use a player out of position.
The trick is to have a balanced squad, If you have a 5* first squad a the subs all 2 and 3* of course you will be in trouble, it is better to have a full 4* squad.
But thats my opinion and the way i play the game.