Anthony Mason, a cornerstone of the tough-minded Knicks teams that came close to winning an N.B.A. championship in the 1990s, was in a New York-area hospital on Thursday, battling the effects of a serious heart attack that appeared to put his life in jeopardy.

Don Cronson, who was Mason’s agent when he played in the N.B.A., expressed slight optimism during the day on Thursday after a member of Mason’s immediate family gave him an update on Mason’s condition. “Right now, it’s day to day, but that’s an improvement from moment to moment,” Cronson said.

He said Mason, 48, who last played in the N.B.A. in the 2002-3 season, had gone to a hospital for a checkup for heart issues and had the heart attack there. Cronson said family members had been told that Mason might need a heart transplant if he survived.