"Ronnie has a serious problem in the tendon of his left knee, so it is not the one that underwent surgery in the past," admitted Milan vice-chairman Adriano Galliani at the final whistle.
"To see him cry like that, it affects those who are on the pitch and the directors. It was a terrible feeling."
Ronaldo had been an injury doubt for this game and was introduced as a substitute in the second half, but lasted only three minutes.
"Ronaldo is in hospital and over the next few hours we hope to understand what has happened," added Ancelotti.
Il Fenomeno had been spotted before kick-off receiving massage around the knee area, but that was again on his right leg - the one that had undergone surgery twice in 1998 and 2000.
"The problem he had was on the other leg and there was no indication of anything serious. If he played it was because he could, we're not crazy."