Okay. Complete breakdown (thanks, ESPN for having the best game log around for this purpose, made it much easier to quickly compile this... can't believe I'm thanking ESPN, anyway...)
Boston figures first.
1st goal of the season put us up 2-0 over Tampa.
2nd goal we were down 3-2 to Tampa. Tied it (ended up losing 3-6).
3rd goal made it 2-1 against Dallas.
4th goal made it 3-5 (the final score)
5th goal made it 1-0 against MTL (we lost 2-4)
6th goal made it 3-2 against San Jose (we lost 3-5)
7th goal made it 2-1 against Minn,
8th goal (GWG) 3-2, and
9th goal 4-2 (ended up 4-2)
10th goal made it 2-1 against Nashville (lost 2-3)
11th goal (shorthanded) made it 1-1 against Montreal (won 3-1)
12th goal made it 3-2 against Pittsburgh, GWG (won 6-2)
13th goal made it 1-0 against New Jersey (we tied 1-1 and won in a shootout)
14th goal made it 1-2 against Washington (we lost 2-3)
15th goal made it 2-1 against Philadelphia (we lost 2-3)
16th goal made it 1-0 against Buffalo (we won 2-1)
17th goal made it 2-1 against Winnipeg (we won 6-2)
18th goal made it 3-1 against Minnesota, GWG (we won 4-2)
19th goal made it 3-1 against Detroit (we lost 6-5)
I marked the goals he scored when we were down in bold (a total of 4 out of 19)
One problem with your reasoning is it would mean he wasn't clutch in the Minnesota game. He scored three of four goals for Boston, including the game winner, but none of them came when Boston were down. Two came when they were tied, and one when it was still a one-goal game, but you never mentioned either of those scenarios. Hockey is quick, every goal counts and even a 5-0 goal could end up being the game winner (unless you have 2 min remaining in the game, where it would be quite unlikely). All of this isn't even taking into account when his assists were made (I don't think it necessary to bring that up, because assisting on goals when you're down isn't necessarily more clutch than when you're tied or ahead)
Just as scoring to make it 6-0 late in the game is not "clutch", scoring to make it 1-5 late isn't either. Scoring to make it 3-3 with a second remaining might be clutch, but so is scoring a GWG (Marchand scored a game winner on a penalty shot in OT, but we weren't down a goal, so he didn't put in an effort that night!) or making it a two goal game with a couple of minutes remaining after huge pressure from the opposing team.