It resulted in a pretty nasty cut, some blood, and a stoppage in play just as Thornton was ready to attempt a shot on goal, negating what would have been a scoring chance.
The latter part of that sequence did not leave Bruins fans in a good mood, and they made their feelings known until Shaw left the ice, skating off under his own power.
I'm assuming Bruins fans were booing the fact the play was stopped when Thornton appeared to have a quality scoring chance and not the fact that Shaw was injured.
It's also worth noting that Shaw is one of the few young players in the NHL who does not wear a visor. Starting next season visors will be grandfathered into the league, largely because of situations like this.
Shaw returned to the game to start the second period.