Okposo played two games after returning from a rib injury that caused him to miss 10 games. He became ill prior to a game against the Columbus Blue Jackets on March 28.
"It's a difficult thing for him to be dealing with now in this situation," Sabres coach Dan Bylsma told the Buffalo News at the morning skate Wednesday. "Very, as a coach and a person, concerned for him and his situation. ... He's seeing our doctors and done some tests."
Bylsma did not have an update after Buffalo defeated the Montreal Canadiens 2-1 at KeyBank Center on Wednesday.
Okposo had 45 points (19 goals, 26 assists) in 65 games this season, and 414 points (158 goals, 256 assists) in 594 games with the Sabres and New York Islanders.
"You always feel for a teammate when he's not feeling well, especially a guy like Kyle who brings a special element to our team and our locker room every day," Sabres forward Jack Eichel said. "Obviously, he's a special player on the ice that we definitely miss. It's never easy to see a guy that you care so much about, and we all care [a lot] about Kyle, it's never easy to see him go through what he's going through, not feeling well.
"We just all want to be here to support him and wish him all the best. Hopefully he gets healthy soon."