Petry has been out of the lineup since suffering the injury in a game against the Carolina Hurricanes on Feb. 7. He had missed the Canadiens' past seven games.
He had recently returned to practice but had been termed day-to-day before Thursday's announcement.
Petry, 28, has five goals and 11 assists in 51 games this season.
The Canadiens also announced Thursday that defenseman Tom Gilbert will be out at least six months after having knee surgery. Dr. James Andrews performed the procedure on Wednesday.
The 33-year-old Gilbert had one goal and one assist in 45 games this season and suffered the injury on Friday against the Philadelphia Flyers.