Like everybody else has said, hopefully Pierce would step up. One thing that's scary about this, though, is Rondo isn't a good enough shooter to force Lebron to stay with him. Because of this Lebron could cheat off of Rondo and play "free safety" like Kobe has in the past. This would allow Lebron to double Pierce or get head starts on the fast break.
This is the real issue here. Lebron will NOT be guarding Rondo, he'll be
sagging off of Rondo - HUGE DIFFERENCE! Cleveland's game plan will be to dare Rondo to shoot a low percentage shot 15-20 foot shot. Even when Boston will try to do a pick and roll, the defender will almost always go under the screen to dare Rondo to shoot.
I'm sure that's their gameplan. What will we do then?
Edit: also on offense if Perk is in the game and LeBron is sagging off of Rondo, you might as well call it a 5 on 4 game. (the 4 is Pierce, Allen, KG and 1/2 of Rondo due to shooting and 1/2 of Perk due to stone hands.)
Cleveland will try to limit touches for KG, Pierce and Allen and force either Rondo to shoot or Perk's offense in the paint.