In fairness though Cina, Rooney playing that role is different from Kagawa in many ways. I don't doubt Rooney's stats would be better, but would the team be better is the question?
I think Kagawa would provide a level of control that Rooney can't in the final third. Rooney there would probably get better stats than most players, but we watch him play there and get 2 assists to his name and we moan about his touch, his composure, his ability to pressure.
Although the same position, I think they'd play the role in completely different ways, Rooney looks to spread the ball out wide whereas Kagawa aims to keep the ball around the box until an opportunity comes to play someone behind.
I definitely think a 3 of Nani, Rooney and Kagawa could work - with all 3 swapping positions and interchanging though. I think Kagawa could play on the left OK under Moyes as long as he is given some freedom to tuck in and bring the ball inside.