Milan striker Ronaldo could be out for nine months after rupturing a tendon in his knee against Livorno.
Ronaldo had only been on the field as a substitute a matter of minutes when he fell awkwardly to the ground on the hour mark.
He was stretchered off in tears and scans have shown that the former Real Madrid ace has ruptured the tendon in his left knee.
Though Milan are unsure how long Ronaldo will be sidelined for, vice-president Adriano Galliani acknowledged it is a 'serious' injury.
"It's a problem with the tendon in his left knee, not the knee that was operated on in the past," Galliani told Italy's Sky TV.
"The severity of it isn't known yet, but it's a serious problem."
Ronaldo's future at Milan will now be in severe doubt as his contract expires in the summer and he had been challenged to prove his fitness in order to secure an extension.
He had been doubtful for the 1-1 draw with Livorno after picking up a knock against Siena, but coach Carlo Ancelotti says this was not a contributory factor in the 31-year-old's latest setback.
Ancelotti said: "His previous problems were with his other leg, if he played it's because he was able to do so, we're not crazy."