Or you put him on and then he has to come off and you’ve used all your substitutions so you’re down to 10 men. And you use two substitutions instead of one. Whereas, with my way you only use one!
If they’re injured then don’t have them in the squad at all. How someone can give you 30mins seems odd to me. They’re either injured or not, so not sure how you’d know if they can give 30mins or not.