Former seven-time world champion Schumacher suffered head injuries in a skiing accident in the French Alps in December 2013 and spent six months in an induced coma before returning to his home in Switzerland to continue his rehabilitation.
"I get news about him constantly, and unfortunately it's not good," said Montezemolo when asked about Schumacher in Milan on Thursday.
Montezemolo did not elaborate on 47-year-old Schumacher's condition, but he added: "He was the most successful driver Ferrari ever had. He only suffered one accident in his entire career, in 1999, and it was our fault, not his.
"Unfortunately, a skiing accident ruined his life."
Amid speculation Schumacher was at fault for the accident that has left him in a coma for over two years, Montezemolo defended the former champion.
"It's not true that Michael wasn't careful on his skis. When he went off piste, he was always careful," he added.
"More than anything, on skis he was very careful."
Schumacher's manager Sabine Kehm declined to comment on Montezemolo's remarks when contacted by British TV channel Sky Sports.
Kehm's most recent official statement was issued last May when she said: "We're glad to be able to continue to say that he is making progress, as I always say, given the severity of his head injuries".
In November, Schumacher was described by FIA president Jean Todt as "still fighting".
"Michael was and remains a close friend of mine," said Todt, who was Schumacher's team boss at Ferrari when the German won five titles between 2000 and 2004.
"His family is very close. I see Michael often and can tell you: Michael is still fighting. And his family are still fighting by his side."