I have a habit of NOT using red packs so my stock is in the hundreds (now 550+)
I think there is a value that dictates injury rate, it won't recover that easily so if you heal an injured player using reds there is a high chance to get injured again unless you use a lot of greens too to re-train him back to shape.
Cheapest way is just let him stay at the reserve but should have a good sub for him.
If he is so important in your team, you have to blow up a lot of reds and greens, there is no other choice.
It is not only your own problem. The ability to field key players in important games is a key element in resource management.