I think he has made a difference. The Celts needed somebody who could fill the role Amir has occupied this year. A long, mobile big with decent height who can defend multiple positions, stretch the floor a little, set picks, roll to the rim, catch and finish. He does a lot of the same things Bass did, except he's not so undersized and he's a much better passer.
That said, Amir is on the decline, and his rebounding overall is not great. What the Celts need is somebody younger who can do a much better job filling that role. Similar skillset, but better overall skills and talent, with the ability to play 30+ minutes on a regular basis.
Too bad they didn't draft Bobby Portis .... though perhaps Mickey can step into the role.