The 32-year-old forward has missed three games because of the injury, which Stastny sustained during a 4-2 loss to the Buffalo Sabres on Oct. 8. He has 10 shots on goal in three games.
"He's going to be out longer than we thought," Gallant said.
The Golden Knights also will be without defenseman Deryk Engelland and forward Alex Tuch against the Sabres at T-Mobile Arena on Tuesday (10 p.m. ET; ATTSN-RM, MSG-B, NHL.TV).
Engelland left a 1-0 win against the Philadelphia Flyers on Saturday 4:51 into the second period because of an undisclosed injury.
Gallant said Brad Hunt will replace Engelland against the Sabres. Hunt was a healthy scratch in Vegas' first six games. He had 18 points (three goals, 15 assists) in 45 games last season.
Tuch (lower body) skated Monday but will miss his seventh straight game. Gallant said he will be re-evaluated after the next couple of games.
