No one said anything about him being a main cog, although by the way the league is these days, and depending on an individuals definition of "main cog", a 3rd option most definitely can be considered a main cog. If you are talking about him being the centerpiece, of course he's not fit for that. But if you are talking about him being one of a group of centerpieces, he certainly can fit that bill.
Regardless, the only thing said was that he can be more productive, and meet higher expectations when Monta and Rondo aren't dominating the ball 98% of the time.