Rondo's weakness on defense has always been getting around picks, that's not going to change overnight. The Cs big men are known for being some of the best at defending the pick and roll by hedging and then getting back into the play quickly. Stopping Jameer is all about the team defensive scheme, and KG and Perk doing what they can do to thwart the pick and roll with the help of their teammates who momentarily cut off the passing lane to the rolling Howard.
That said, they could use some energy off the bench to bug Jameer while Rondo is resting. With TA hurt, Doc could turn to Nate because he looked good for short stretches on defense. When Eddie House was here, the one thing he could do better than Rondo on defense was fight through picks. Maybe Nate can come out and play like Eddie in that sense.
Rondo would never guard Reddick or Barnes though, I'm only talking about while Rondo is on the bench. Barnes would post him up too easily and Reddick loves curling off screens and Rondo sticks to screens.