A little clarity
Boston has played 74 games.
Green, Terry, and Bass have played in all 74.
Rondo has missed 36.
Barbosa has missed 33.
Bradley has missed 31.
Sullinger has missed 29.
Wilcox has missed 20.
Garnett has missed 9.
Lee has missed 3.
Pierce has missed 2.
Now about the New York Knicks.
New York has played 72 games.
JR Smith is the only player to have played in all 72. Steve Novak played in 71.
Amare Stoudemire has missed 43 games.
Iman Shumpert has missed 37 games.
Kurt Thomas has missed 33 games.
James White has missed 22 games.
Carmelo Anthony has missed 13 games.
Raymond Felton has missed 12 games.
Tyson Chandler has missed 10 games.
Jason Kidd has missed 5 games.
Pablo Prioni has missed 4 games.
Rasheed only played in 20 games all year and Kenyon Martin and Marcus Camby were late in the year signees to replace injured players.
The Knicks haven't been affected by the rash of injuries like the Celtics have. Frankly because they have a better constructed and deeper roster and have been willing to make the moves necessary to improve the team (like trading Ronnie Brewer for a 2nd rounder to open up a roster spot for a late in the year signing).
The Knicks have a superstar who carried them for much of the season, and a DPOY candidate who played much of the season, too.
It's not really a valid comparison.
I do agree with your implication that the Celtics have still underachieved this season despite the injuries. The injuries are a valid excuse to some extent, but the team had some really bad stretches even when they had most of their guys healthy.
The Celtics are one of the biggest underachievers of this NBA season, no doubt about it. That, coupled with the injuries, is why a team with the talent to win 55+ games is going to struggle to finish above .500.