Maybe I'm biased from watching Man in the Arena, but to Belichick, this loss has to be a little like the Week 7 loss to the Steelers in 2004 that snapped the 21 game win streak. Obviously that was a better opponent and much longer streak, but that game was bad in every way - they mustered all of five rushing yards, gave up four sacks, fumbled 3 times and lost two of them, and Brady threw two picks. On other other side of the ball, they gave up 221 yards rushing, with Staley and The Bus running all over them.
That 2004 team needed the bubble to burst in order to move forward. I'm sure this team, and Belichick, would rather win than lose, but I wouldn't be surprised if Belichick views this game as a much needed reality check, rather than a sign of the team being fatally flawed.