McAvoy underwent a left shoulder arthroscopic stabilization procedure in Boston and is expected to miss about six months, the team said. The former first-round pick posted 10 goals and 56 points this season.
Grzelcyk underwent a right shoulder open-stabilization procedure, also in Boston, and he's expected to be out five months, per the team.
In addition, defenseman Mike Reilly had a procedure done on his right ankle in Wisconsin that is expected to keep him out for about three months.
The Bruins already were expecting to be without Brad Marchand for the beginning of the 2022-23 season. He had hip surgery last week, a procedure that is expected to keep him out six months.
Boston lost in the opening round of the playoffs to the Carolina Hurricanes.