Rafa, the coaches, Joselus teammates and some supporters.
If Shelvey see that Mitrovic is being picked ahead of Joselu when Joselu out performs him in training, maybe he too will put in less effort because he knows he will get picked anyway. Lascelles see that Shelvey and Mitro are picked ahead of Joselu and Merino/Hayden and puts in less effort etc etc. Joselu gets angry that he is not picked and stops making any effort because why should he if he isn't getting picked anyway. Poor training results in poor match performances and we go down.
One of the few advantages we have this season is fighting attitude and team spirit. I don't see why Rafa would risk spoiling that in order to accommodate one player.
This is the truth.