I'm going to say Kevin McHale. The key to this series will be shutting down Gasol inside, and McHale could do that. Also, McHale could score consistently on the block; I don't think anybody on the Lakers could stop him.
I didn't pick Russell because he doesn't offer the offense that McHale does, and he'd be somewhat undersized in today's NBA. Larry is my favorite Celtic of all-time, and he could help *any* team win, but I don't think he'd be quite as good of a fit for what we need right now.
Larry would probably be my second choice, and Cowens may be my third.