Horton had 32 points in 46 games last season, but didn't play after suffering a concussion from a hit by Flyers forward Tom Sestito on Jan. 22. It was his second concussion in six months.
Horton was also hit by Vancouver Canucks blueliner Aaron Rome in Game 3 of the Stanley Cup Final in June of 2011 and didn't return until the start of the following season.
McQuaid was injured on March 29 after a hit by Capitals forward Jason Chimera. It was later reported after the Bruins were eliminated by the Capitals in the first round playoffs that McQuaid had a concussion.