The hit took place Tuesday on a play away from the puck. Ristolainen received a five-minute penalty for interference and a game misconduct.
Here is the play in question, courtesy of SportsNet Canada:
The 22-year-old has played in all 74 games for the Sabres this season, totaling five goals and 39 assists.
Pittsburgh coach Mike Sullivan noted Guentzel, who has 25 points in his first season with the Penguins, was diagnosed with a concussion.
"At the moment, I didn't expect to get thrown out of the game but later on he obviously didn't have the puck," Ristolainen said of the hit, per Amy Moritz of the Buffalo News. "He didn't get it. Our player broke the play and he got injured. So, I don’t know what are the rules but it probably was the right call. I'm not aware of the rules."
Pittsburgh has already clinched a spot in the postseason while Buffalo sits just one spot out of last place in the Atlantic division.