Because he is overusing same players constantly. Where was the problem to start Asllani or Gagliardini instead of Barella/Mkhitaryan in a game like this? This way, one has a spare quality midfielder on the bench and 2 decent on the field. Same with Dzeko. The guy is 36 years old and it is clear that he cannot deliver every 3-4 days. Lukaku has low confidence and he kept him on the bench? For a mentality fragile player like him, that was bad decision, and considering that Dzeko played almost full game couple of days ago, he should have come like a substitute instead.