Gaustad, 29, was placed on IR on December 4 with an upper-body injury after being injured in a 3-2 victory over the Nashville Predators on December 3. Gaustad has registered one goal and six points in 26 games this season.
Weber, 24, is active for the first time since being injured in a 5-3 loss to the New Jersey Devils on November 16. Weber has played in only three games this season, recording seven penalty minutes and a minus-5 rating.
The Sabres also placed forward Nathan Gerbe on IR.
Gerbe suffered a concussion in the team's 5-4 overtime loss to the Philadelphia Flyers on December 7. In 26 games, Gerbe has registered three goals and 12 points.