Dorsey isn't really a replacement for Stiemsma. Playing him at center doesn't work too well.
Dorsey needs to play PF alongside a highly skilled center who can create proper spacing for the team as a whole despite Dorsey's limited offensive game. Be the Varejao to someone's Zydrunas Ilgauskas. That type of role is what Dorsey needs to be successful in the NBA. Garnett offers that, the backup centers (Fab Melo, Stiemsma, Hollins, S.Williams) do not.
I'd like to bring Dorsey in as a backup PF. He offers a very different skill-set and quality alternative to the Celtics current options (Bass, Sully, JJJ, Green) there.