He hit the ground running since his first game for us (it was something ridiculous, like 4 MotM performances in a row? edit: only 2). Mourinho is a brilliant coach, but even he couldn't have coached him that well in those few weeks that they worked together. He probably inspired confidence in him, that's true - plus seeing as it was his personal approach that got us Bailly in the first place
Mourinho is the right manager for him in the long run though, and he'll definitely benefit from working with him