The original post is a nice message, but I think it too is an oversimplification.
The Celtics are a very good road team - the best in the league - and have been all year. However, they've been disturbingly bad at home this year; they've already dropped 11 games at home versus 6 all of last year, and they are ranked 15th in the league for home record. That disparity can't be explained away by injuries.
Also very troubling is the fact that the Celtics only have a handful of significant wins against really good teams - all on the road. They beat Cleveland in the season opener, the Magic on Christmas, and the Lakers a few weeks ago. The only big home win I can think of is a big win over the Jazz in November, though at that point in the season the Jazz couldn't be considered a top team.
What the Celtics do have is a lot of tough, sometimes embarrassing losses to good teams that they could have and should have beaten if they want to consider themselves contenders. Early in the season it was the disheartening home losses to Phoenix, the Hawks, and the Magic. Then it was getting beaten by the Hawks 3 times in January for the season sweep, coupled with two crushing breakdowns against the Magic and a late-game choke against the Lakers.
Again, I would argue that these problems can't be explained simply by injuries, because the Celtics have consistently given up leads and had difficulty beating good teams throughout the season even when they've been at or near full strength. I don't think you can explain away all of those blown leads simply by pointing at the injuries to Pierce and Garnett. The fact is, even when Pierce or KG were out or somewhat hobbled by injuries, there were games this team could have and should have won, but failed to win (e.g. the most recent game against the Cavs). Injuries cannot account for the Celtics' lack of focus, and their inability to hold a lead and consistently play the kind of smothering defense they used to play pretty much all the time.
Last year, the Celtics lost KG for basically half the season. Despite that, and despite having an even thinner bench than they do this year, they fought hard and won a lot of games they probably shouldn't have won. They ended up winning 62 games. This year, the Celtics have been hit with injuries, but it's not as bad as last year. Knock on wood, all of the C's starters have been able to play for the majority of the season, and they have a much stronger bench than last year. Rondo has been playing great for most of the season - much better than ever before. The Celtics should be very good this year, far better than last year. Nonetheless, they have not fought hard and they have lost a lot of games they should have won. As a result, they'll be lucky to get 55 wins.
Whether or not that's because of a lack of energy, athleticism, and intensity due to age or boredom, it doesn't really matter. The bottom line is the Celtics haven't played as well as they ought to have over the course of this season, and injuries alone can't explain it. Whether the Celtics will have it all together for the playoffs remains to be seen.
The "pessimists" on this board are being more "realistic" than you give them credit for.