Depends on how you look at it. If you're talking about making the playoffs, this game isn't a "must win," as the Pats could lose it and still win out in what is turning out to be a mediocre division except for New England.
From another perspective, though, since the Pats came through the first part of their season in pretty good shape (5--2, considering Brady's return, a revamped D and the usual injuries), my own hopes and expectations are very high right now for this team.
There's a lot of agreement out here that the next five games will tell us how good this team really is. Starting those games by losing at home, after a Bye, to the 3--4 Dolphins would suggest that I might want to think about lowering my expectations and would also probably make the Pats at least a six point dog in the Colts game.
So, if I'm going to continue thinking about this team as contending for a championship, this Sunday is a "Must" win. But, since they are still strongly in the hunt for the playoffs if they lose, in that sense it isn't a "Must."