Lukaku was sold because Mourinho did not want him.
I am sure he would have stayed if he was going to be a first team player.
Mourinho did want him he just didn't want to put his trust in a 21 year old which like Deulofeu is totally understandable but he wanted him in the squad. In fact Mourinho sounded very bitter that he left.
That's the problem at top clubs, the young players need game time and they don't get that. It's a great move by Deulofeu, shame he isn't coming back to Everton but it's good for him and it's a great move by Lukaku as well.