The reality is that Rondo averaged nearly 37 mpg last season during the regular season. In the playoffs he averaged nearly 43 mpg.
Bradley only put up 21 mpg as an average, but put up nearly 33 mpg in the month of March when he was on his tear.
Jason Terry put up nearly 32 mpg last season in Dallas.
Even if you were to conservatively estimate that Rondo will paly 35 mpg and Bradley will only play 28 (and that's very low IMHO, since he's so young), that only leaves 33 mpg of backup minutes between Rondo and Bradley.
Even if we were to adjust Terry down to 28 mpg, that only leaves 5 mpg for whoever the backup is.
And considering that the C's need to create some more minutes for Green at the 3, I wouldn't be surprised to see Pierce slide down to the 2 for those leftover minutes.
The end of the story, assuming everyone's healthy, the backup 2 guard spot isn't a huge need and becomes practically nonexistent in the playoffs (the only exclusion really being foul trouble).