Rask's season started on an ominous note; after a poor opening night performance, the Bruins went to Tim Thomas the next day and never looked back. It had to be a serious blow to Rask, who led the NHL in both save percentage and goals against average the year before. You certainly wouldn't call his 2010-11 season bad; with a .918 save percentage, he still ranked in the top third of goaltenders (tied for 13th), but his 2.67 goals against was 28th out of 43 qualifying goaltenders, and his 11-14-2 record left much to be desired.
Did Rask just "not know how to win"? Of course not. Rask's #1 problem is that the defense let him down; he faced 30 shots per game. Alas, Rask had the misfortune of being a talented mortal, instead of the godlike figure he was backing up. (Thomas faced an obscene 32 shots per game, which makes his season all the more impressive.) More technically sound, but less acrobatic, than Thomas, he lacks the highlight reel stops to make fans recall his season fondly. Still, Rask generally gave the Bruins a chance to win, as long as he wasn't getting buried in an avalanche of shots. That's what you ask of a backup goalie.
The problem is that Rask wasn't supposed to be a "good backup"; he was viewed as the next great Boston goaltender. His failure to meet those expectations in 2010-11 necessitate a mediocre grade for him.
By the numbers, Rask was probably the NHL's second best backup goaltender last year, behind Semyon Varlamov. That's nothing to be ashamed of, but it's also much less than one would have expected, after Rask's sterling rookie season. To that end, 2011-12 will be crucial for him. Thomas can't play forever, certainly not at his current level, and the transition to Rask will probably happen at some point in the next year or two. If he returns to 2009-10 form (and the defense gets a little more stingy), Boston won't just be a strong team, they'll be a juggernaut with a great chance to win multiple Stanley Cups.