Hours after being signed Saturday, Ruhwedel made his NHL debut, playing in Buffalo's home game against the Philadelphia Flyers. He had 23 points (seven goals) in 41 games this past season in helping Lowell reach the Frozen Four, before losing to Yale in a semifinal game on Thursday.
Overall, he had 63 points (15 goals) in 110 career games for the River Hawks.
The Sabres had an immediate need at defense. Tyler Myers will miss the remainder of the season after breaking his leg in a 5-1 loss to Montreal on Thursday.
Flyers forward Danny Briere returned after missing 10 games with a concussion.