Warriors fan (and GSoM mod/writer) here. I'm writing a team preview for all 30 teams over at my blog, and did Boston a few days ago. Thought I'd share.
First, and most importantly, you should be excited to be a Celtics fan because you still have Kevin Garnett, Paul Pierce, and Rajon Rondo. Garnett and Pierce are inching ever closer to retirement, but it's pretty clear that each is still in the upper echelon of NBA players. Garnett can still defend and shoot long 2's and Pierce is still one of the game's best offensive threats. As evidence, note that Pierce shot 56.7% TS on 28.1% USG. Only Karl Malone has shot with a higher efficiency on that much volume after age 34. The combined RAPM of Garnett (+5.6), Pierce (+1.8), and Rondo (+1) is +8.4. A team with that rating (for a full 48 mpg) would win about 63 games.
Of course, GPR can't play all 48 minutes, so the actual win total will depend on Boston's key role players, including the returning Brandon Bass and Avery Bradley, newly acquired Jason Terry, Courtney Lee, and Jason Collins, and to some extent the rookies Jared Sullinger, Fab Melo, and Kris Joseph.
Let's start with the returning players. Brandon Bass is 26 years old and should be entering his prime. He's a very good mid-range shooter, in fact, right up there with Garnett himself. Bass shot 45% on 2-pt jump shots. Unfortunately, he doesn't have 3-pt range, so while that 45% is (extremely) impressive for a mid-range shooter, it doesn't have quite the impact that, say, Ryan Anderson or Matt Bonner seem to have on floor spacing. And to a large extent, Bass' mid-range shooting strength overlaps with Garnett (and Terry actually). Overall, Bass' TS% dropped from 57% all the way down to 52.4%, indicating that he took a big hit moving away from Orlando (and Dwight Howard). Looking more closely at the numbers, it appears that his efficiency on layups might be the culprit. In 2011 he shot 53% on 96 layup attempts. Last season he shot a woeful 33% on 117 layup attempts. I would call that a fluke, but it's a fairly large number of attempts. This is worrisome and I'm sure it's something that Bass and his coaches are aware of. Bass' +1.1 defensive RAPM suggests he can hold his own on that end of the floor. I think it's fair to say Bass is an average starting PF, and for Boston, that's a pretty good deal and about what you'd expect for a guy making $6M per year.
Avery Bradley, still on a valuable rookie contract, will have a big role on this team assuming he returns 100% from shoulder surgery this off-season. If we look at units last season when Bradley and Rondo were playing with the same 4 teammates, the units with Bradley were only 1 point per 100 possessions worse than those with Rondo. (I call this metric "Same 4 +/-" or S4PM for short.) Not surprisingly, those units with Bradley, while quite a bit worse on offense, were much better on defense, leading to the net result being almost a wash between them. Bradley's overall RAPM is -0.4 (slightly below average), again the result of less than stellar offensive impact (-1.7 ORAPM), combined with well above average defense (+1.3 DRAPM). My own adjusted +/- metric (based on the four factors) also points to Bradley's prowess (he ranked 4th among all players with fewer than 3000 possessions). If Bradley improves on the offensive end to the point where he becomes just average, he will be extremely valuable whether he continues to be a Celtic or, perhaps, is part of a major trade.
Now let's talk about the new guys, starting with Jason Terry. Jason Terry was brought in to essentially play the same role he did in Dallas since 2005, namely providing offensive firepower off the bench. The question I have is whether he can replace Ray Allen, one of the game's all-time best shooters. What concerns me about Terry is that his TS% has been steadily diminishing for several years now, from 60.6% his first year in Dallas all the way down to 54% (league average) last season. Hopefully for Boston fans, Terry reverses this trend. Overall, Terry (-0.6 RAPM) may be a step down from Ray Allen (+0.6 RAPM), but I think he'll be a "good enough" replacement.
Houston had to make some difficult choices this off-season, and one of them was to let Courtney Lee walk. That might end up looking like a huge mistake eventually. Lee is just entering his prime. His S4PM compared to either Kevin Martin (+8) or Chandler Parsons (+3) was significantly better last season. His +0.7 RAPM (+0.9 offense, -0.2 defense) suggests he is an above-average player on a relatively cheap second contract ($5M per). Lee is a 40% 3-pt shooter, and is actually better from behind the line than just in front of it (35.6% last season from 16-23 ft). As a wing, Lee should be a good spot-up target for Rondo.
The rookies. Jared Sullinger and Fab Melo could be significant contributors to Boston's success, providing interior offensive and defensive depth, respectively. Or not. Obviously, if either were "locks", they never would have dropped to Boston.
Boston is going to be good. The question is how good and for how long. Nobody's picking them to finish ahead of Miami (actually, that's not quite true, but those people are crazy), and I'm not either. They have as strong a chance as any team in the East to make it to the ECF and push Miami, but I honestly can't see them winning it unless everything clicks, meaning no significant injuries, All-Star caliber seasons from each of the Big 3, and major productivity from the new additions. All those things happening together are possible, but not highly probable.
If you're a Boston fan, I don't need to tell you that this team is nearing the end of an era. No matter what happens this year or maybe next year, once Garnett and Pierce leave, it may take years for Boston to get back to this level of competitiveness. So enjoy it while you can and worry about tomorrow...tomorrow.
Notified by @vypahn via Twitter that I neglected to mention Jeff Green. HOW COULD I HAVE FORGOTTEN ABOUT JEFF GREEN!? After a heart problem made him miss the entirety of last season, he's back and in a backup SF role that is probably his best hope for career improvement.