"I don't know if it has torn or what has happened," the tearful Internacional forward told reporters at half-time during the 1-1 draw with Figueirense after going off clutching his right thigh.
"I felt a sharp pain. I want to be back as soon as possible," added Damiao.
The 22-year-old, who scored his first goal for Brazil this month in a 1-0 win friendly over Ghana, has scored 39 goals in all competitions this year.
Brazil host Argentina next week, a match in which both teams will field only home-based players, and play Costa Rica and Mexico early in October.
A goalless draw against arch-rivals Corinthians was enough to take Sao Paulo level on 45 points with Vasco da Gama at the top of the Brazilian championship, although Vasco have a game in hand against Atletico Goianiense on Thursday.
Corinthians, who fined former Inter Milan and Brazil striker Adriano after he was late for training on Monday, are a point behind in third place.
Adriano, who has been sidelined with injury, denied in a television interview that the club had threatened to fire him.
Ronaldinho scored to give Flamengo a 1-1 draw at Atletico Mineiro, although his side stayed sixth with 38 points after completing their 10th league match without a win.
The former Barcelona and AC Milan player scored with a deflected shot on the hour after Daniel Carvalho had opened the scoring for the Roosters directly from a free-kick.
