They are knackered due to heavy training sessions ongoing. It is no worry to play a game in there but they will not be as fresh as we can expect them in some 4-5 weeks. Fabs is fit and fine and will be in the team, no worries there. The rotating is for later stages of the season when the engine is burned and the amount of training is limited due to heavy program, which is clearly not the issue right now.
You make it sound like a typical situation in youth sport over here, where it is a trend of parents overprotecting their kids from any kind of stress. They can't train if they look stressed and the coaches are told by parents how things should work. They should not practice at certain times and not more than 5 times a week and they surely can't handle to much homework in the process. They look stressed and knackered if they have to make their own food. So parents brush away any disruption in front of them so they can walk around in life looking fresh and healthy all the time, which they clearly don't so it freaks them out even more.
In my opinion all players should look knackered after a game. At least after a final. They are paid good money to run their bodies down and look sweathy and knackered at the end of games. That is what we should expect from them. If they, and in this case Fabinho who has been free to build form since May, needs rotation after two games he should be sent to hospital for serious examination.