To be honest, I think there is close to zero chance of Rondo ever averaging 20 PPG.
Why?
Rondo's problem is that his offensive versatility (from a scoring perspective) is very limited.
His effective options are basically either:
a) Drive to the basket for a floater / layup
b) Pull up mid-range jumper
Option 'a' is not usually effective unless you're playing a team that lacks rim protection, and option 'b' is officially the most inefficient shot in the game.
He can get points via these shots, but it's not sustainable. Not to the point where he can score at will and be a nightly 20 PPG threat.
I think as a scorer, Rondo's ceiling tops out at maybe 15 - 17 PPG. Realistically, he'll probably be more around the 13-15 PPG mark, around the upper level of where he's been in past seasons.
I understand the rationale that now the big 3 are gone and so Rondo has more pressure on him to score, but at the same time you need to consider the fact that:
* Ray Allen left the team a couple of years ago
* KG was not the same player offensively his last two seasons in Boston
* Pierce was nice the same player offensive in his last season in Boston
In their past couple of years together, the Celtics team was one of the worst in the entire league in terms of offensive production. It's not like they were a team full of high volume scorers, and so they never needed Rondo's production on that end of the floor. They badly needed scoring from anybody and everybody, but Rondo never really managed to average more than around 14 PPG on a consistent basis.
Another thing people will say is that now our team is full of skilled passers, so Rondo can get more quality looks. Again, I question whether the duo of Sully and KO are more skilled passers then Pierce and KG? I would suggest not.
I think Boston is very much looking like a San Antonio type team - a team that won't have anybody scoring over about 17 PPG, but everybody on the court will be a threat.
Ironically, I actually think Olynyk, Smart and Young are the only guy on this team who have the potential to one day be a 20 PPG scorer. Maybe Evan Turner. I don't think anybody else on this team (not Rondo, not Green, not Bradley, not Sully) has that potential.