"I think with Paul, I watched him come through the academy at United and he's got a lot of ability," Rooney said, speaking to ESPN.
"If you don't have a good relationship with the coach, then it's difficult to be at your top if you feel like the coach is scrutinising every decision you make, every pass you make.
"Paul Pogba will give the ball away, he'll try passes [and] it won't be the right pass at times.
"But you have to let him do that because two or three of them passes will create goals, will create opportunities.
"I think he found it difficult under Mourinho.
"I think a player and a manager with big egos clashed and the outcome was never going to be great."