I have no doubt this team would be better with Rondo, especially in the postseason.
Nonetheless, what I've seen in this series and throughout most of the regular season is players simply missing lots and lots of open shots—threes, midrange, and even layups. Terry's been off all season, Lee and Bradley for good chunks of the season, Pierce has often struggled.
Doesn't matter much who your point guard is if you can't do something so basic as make open shots.
Jason Terry was basically the exact same player this year that he was last year. In fact, he has basically been the same player the last 4 years or so, he just had less shots, but was hitting almost an identical percentage of them.
Courtney Lee, basically the same player as well. So was Bass. Pierce was also pretty much what he was last year this year. Green actually had his most efficient or second most efficient season in his career. Garnett faded a bit this year, but that just matches his trend of the last 4 years.
In other words, Boston's players performed exactly like they should have. they just aren't very good, especially without Rondo. I've said it before and I really think this is true, last year was a fluke that blinded the fan base into thinking Boston was better than it was. I think if Horford was healthy last year, the Hawks might have beaten Boston in the first round, and certainly if Rose was healthy, Boston would have lost to Chicago in the second. The Celtics just aren't that good.