The rookie first-round draft pick was sent down to AHL Rochester on Saturday, before Buffalo's 4-2 loss to Colorado. The Sabres made the move to free up a roster spot to activate fellow rookie defenseman Nikita Zadorov off injured reserve.
Ristolainen, drafted eighth overall in June, has no points in eight games this season.
At 1-8-1, the Sabres are off to their worst start in team history. They close out a three-game homestand against Boston on Wednesday.
Rookie forward Corey Tropp practiced for the first time Monday since breaking his jaw in a fight during a preseason game against Toronto on Sept. 22.