My take on a KG absence vs. a Rondo absence:
If Rondo went down tomorrow for the year but everyone else were guaranteed to be healthy for the season, the Celtics win 55 - 60 games and challenge for the title. (This isn't hyperbole...I can even tell you how I think it would go):
*Starting Bradley, Lee, Pierce, Bass and Garnett the Celtics have the #1 defense in the NBA and the best since 2008
*KG still does his 5-5-5 plan, meaning that there are three distinct units that get run. The first unit features Pierce as something like a point forward (averaging at least 5 assists per game) and the leading scoring option, followed by KG with Bradley/Lee/Bass as shooters/cutters that play off of them. We've seen variations of this unit in limited action in the 27 games Rondo has missed in the past three seasons. It has been successful in short stints...I believe it would be successful over larger periods.
*The second unit features Terry, Bradley (or Dooling), Green, Sullinger and Garnett. The main set for this unit is the Terry/Garnett pick and roll/pop (which, by the way, you are going to LOVE this year. Most of you have never seen KG in a true pick-and-play pairing. You will enjoy it). Green will be the primary weak-side scoring threat opposite the pick-and-play, and Sullinger will be the garbage man/post outlet off of the main action. Bradley (or Dooling) will essentially spot up in the corner.
*The third unit is the one that will close halves and games: Terry, Lee (or Bradley), Pierce, Bass (or Green or Sullinger), and Garnett. This will be the most dynamic unit on the team, with 5 legitimate scoring options that all have range out to at least 20 feet. The unit will revolve between Pierce-centric and KG/Terry-centric action, and will be the most dangerous closing unit of the Big Three era bar none.
That team, again with the caveat that no one gets injured, would finish with at worst a top-10 - 15 offense (likely more in the 5 - 8 range) to go with the #1 defense in the league. Championship contender.
If, on the other hand, KG went down tomorrow with the caveat that everyone else remained healthy, the Celtics would finish with 35 - 45 wins, as either a lower lottery or 7th/8th ranked playoff seed.
*The defense would not finish in the top-10...maybe not even in the top-15.
*Rondo would average close to a triple-double, and the team would run a lot. If they stayed focused they would be a competitive squad on a night-by-night basis. But whenever they weren't fully on and/or when they played a good team that was focused they would lose
*By trading season the rumors would be HOT and heavy that Pierce was on the block, and he might even be traded to accelerate a transition to a youth movement moving forward
*The offense would be solid by volume, but would not be very efficient. Starting Rondo/Bradley/Pierce/Bass/Wilcox the offense looks like a weakened version of the unit that we saw last season. Rondo plays 40 minutes per game, so he is in there with the second unit a lot as well (Rondo/Terry/Lee/Green/Sullinger) and that unit is more successful against the opponent's second units. Terry gets to be featured in the 8 minutes or so that Rondo sits out per game, but other than that he is limited to the Ray Allen off-the-ball role that doesn't suit him as well, and he doesn't have a big man to play off of as he has for the past decade.
All told, the fan base would have a superstar to circle the wagons around (as was the case for most years before KG and Ray arrived), and Rondo would not disappoint in the numbers category. He would build his own personal, loyal fan coterie the same way that Pierce did. But the team would be watching the action by May...if not by mid-April.
Conclusion: Now of course, this is one man's opinion so you're welcome to disagree with me. But I ask you, if you disagree...where do you think my above prediction is in error?