The 33-year-old pulled up in training on Thursday and faces up to three months on the sidelines after scans revealed the extent of the damage to his left leg, meaning he could be forced to miss the reigning Copa Libertadores champions' campaign in Morocco, which begins on December 18.
"Ronaldinho felt pain in the left thigh adductor region and, after clinical examination and imaging, we confirmed that he had a total rupture of adductor muscles of the left thigh," medical director Rodrigo Lasmar told Atletico's official website.
"The player is in the care of our physiotherapy department and every effort will be made to ensure that he recovers as soon as possible. We're always very careful in talking about time because there is no rule, there are many factors involved that can influence the term.
"It's a serious injury, but recovery time is short, it revolves around two and a half months, three months, three and a half months. The big question is whether he will be able to play the Club World Cup. We must believe in that possibility.
"A major effort will be made so that it is available to the coaching staff. It is a challenge, but the athlete is motivated, we're motivated and there is no restriction, the treatment can be done full-time."
Atletico are hoping to become the fourth Brazilian side to claim the Club World Cup, after previous successes for Sao Paulo, Internacional and Corinthians, who beat Chelsea in last season's final.
The tournament, which kicks off on December 11 in Agadir, will feature the likes of European champions Bayern Munich, Mexican giants Monterrey and winners of the host nation's national league Raja Casablanca.