The moves were made Thursday before a game at Nashville that night, when the league-worst Sabres (20-44-8) close a five-game road trip.
Ristolainen returns for a third stint in Buffalo this season. The rookie first-round draft pick has a goal in 24 games with the Sabres.
Larsson has also split time this season between Buffalo and AHL Rochester. He has an assist in 21 games with the Sabres.