In this situation I use rests on my fast trainers to get them up to 85+% and then I try to have 3-4 decent depth players (3-5 stars) so that I can rotate players.
But then, if I am at 110-130% or something like that, and face someone that is below 75% or something, I don't hesitate to play a game with 4-6 players around 40-60% fitness. That saves a lot of Rests.