The issue is that they are different sports.
In baseball, the guy with the most ability to control a game is legally only allowed to play once every 3 games and typically only plays once every 5 games. And that guy only plays half of the game (in the AL). i.e. No single player can make a monumental difference on a baseball team and it's not super important to get that 1 guy. So its easier to rebuild the way the red sox have since you don't need to get one of very few players who are difficult to obtain.
In basketball, the guy with the most ability to control the game plays 40/48 minutes of every game, is one of only 5 guys on the court, and has the ball in his hands the majority of that time. Rebuilding in basketball requires getting that one guy. It's SUPER important to get that 1 guy. Then it's very important to get additional top talent around that one guy. Its harder for the celtics to rebuild because they don't have and can't obtain the best players for that one position. If you think Rondo is that guy, well maybe he is, but he'll miss significant time this season. Also, we don't have the additional top talent around him, which is also difficult to obtain when you're over the salary cap.
This is pretty much spot on. Basketball, more than any other pro sport, is a sport dominated by elite players. Roughly 10 players are primarily responsible for most of the championships over the past 30-35 years (Bird, Johnson, Kareem, Jordan, Thomas, Olajuwon, Duncan, Shaq, Kobe, and LeBron). Sure, you get some anomalies like 2004 Detroit, but that was really more the Lakers self-destructing than the Pistons winning. That Pistons team would've been crushed by pretty much any other champion in any other year.
And it's not just baseball that it's different, it's hockey and football too. As great as Tom Brady is, he still only plays 40-45% of all plays (when you count defense and special teams). In basketball, elite superstars can literally play the entire game and have their hands pretty much in every play, offensively and defensively.
So might the Celtics surprise us a bit? Sure. Maybe Rondo is back with a vengeance earlier than expected, Jeff Green proves he's an All Star, and Avery Bradley proves to be the second coming of Joe Dumars or Dennis Johnson. But it won't be because "team" prevails. If it happens, it will because we have more star power than we thought.