I only stayed for the 1st half last night but I when he was in the game he was assigned to Rondo. If that is not impressive for a 6'7", 240 lb rookie, I am not sure what is.
Brad probably wanted to use his length to stop Rondo from getting into the offense. That, and Jaylen is still rather weak defensively. He might play the offensive player perfectly well, but at the last moment, he either falls for a pump fake, or somehow fades back, away from the offensive player, thus giving him a clear look at the basket. He seems to have a little bit of a weak core. Gets pushed around a little too easy. At the time, my guess is that Brad wanted a guy like Avery or Marcus to guard Butler/Wade.
Side note: where is he 240lb? The celtics website has him at 225.