Between Adama and Dembele the one with the better performances in training should start the games and the other one should be his sub. None of these two is guaranteed to be here longer than until summer, so no point in developing one over the other. Ferran is a different case. He should be ahead of those two as he is an integral part of the future.
It also makes no sense for team chemistry. A player who knows that he won't play no matter how good he trains can be pretty toxic for the locker room. Needless to say that he is an old friend of Auba and can help him integrate into the team, although I think this won't be much of a problem as Auba seems to be a pretty cool dude.