Ristolainen, 26, detailed his struggles with significant drops in oxygen levels, intense chest pain, and fatigue to Finnish newspaper Ilta-Sanomat on Friday, according to The Buffalo News' Lance Lysowski and Mike Harrington via Google Translate.
The Sabres had an outbreak in late January and were kept off the ice for 14 days. Blue-liner Jake McCabe, who also had COVID-19, says Ristolainen's scary experience proves how serious the virus is.
"I was texting with 'Risto' and we were both going through it together," McCabe said, according to Lysowski and Harrington. "So obviously, he had a little bit of a worse case than I did and maybe not even a little bit, a lot worse.
"It just goes to show how crazy and serious this virus really is. And Risto is probably the fittest guy on our team. For him to battle like this, it's very tough and unfortunate."
Head coach Ralph Krueger says it will be a "long process" for Ristolainen to return to the lineup because of what he's gone through.
The 6-foot-4 rearguard has tallied two goals and six points while logging a team-high 23:34 of average ice time over 10 games this season.