What I'd do is not let his condition ever get below 70% in a game.
So that means don't start him when he is below 95%, and usually take him off at around 60-70 mins.
If he is your 2nd choice though, you shouldn't really have a problem, as he won't be playing that much football anyway - if you're looking to bring him off the bench for 20,30 mins he should be fine mate