Some people still need to realise that, even if we put Kagawa into his natural position at this stage, he would still take a bit of time to adjust to the game, here. Remember, both Silva and Modric took some time to adjust to the English game (I think both needed 1 full season/changing roles in order to fit in). With Kagawa, Sir Alex initially played him on the left as he was getting bullied out of the ball in the middle areas (see Southampton at St. Mary's, last season). Sir Alex continued to play Kagawa on the left side in order to prevent him from being physically dominated in the middle. Still, Kagawa didn't play in a strict role as he was given the freedom to move around and get involved in the middle areas; however, our team was set up in a crossing system, hence why he wasn't amazing albeit decent and showing promise.
For this reason, Sir Alex believed that he would have a great second season as Sir Alex was looking to ease him into the English game like how Silva and Modric were. However, Moyes came in, and Kagawa had to start all over again with Moyes. If Moyes were to play him regularly through the middle, now, I don't think he'll make an instant impact, but he'll gradually get better and have more of an impact for us as time passes. Still, I'm speculating here, and we'll never know what Moyes will do with Kagawa, at this stage.