Smith, a center who played in both of the Bulldogs' national championship game appearances when Stevens was their coach, has been battling non-Hodgkin's lymphoma. He was scheduled to undergo a bone marrow transplant Thursday, according to The Boston Globe.
Smith's family members have posted requests for prayer on social media in recent days.
With Stevens visiting his former player in Indiana, assistant Jay Larranaga took over the Celtics against the Bulls. Larranaga says he found out late Thursday morning that he was filling in. He was not sure when Stevens will rejoin the team.