Ronaldinho doubtful for Club World Cup
The Copa Libertadores champions will likely be placed as second favourites behind UEFA Champions League holders Bayern Munich for the tournament, which is being held in Morocco for 2013.
However, they could have to do so without one of their key players, with the former Milan and Barcelona star likely to remain sidelined for the next three months.
Club medical director Rodrigo Lasmar told Atletico's official website: "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.
"He's in intensive care and we await developments. 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."
MRI images displaying Ronaldinho's injury had been leaked to the Brazilian press with those actions condemned by Atletico in a separate statement.
Lasmar wrote: "The leak of these pictures is absurd and brings outrage as it is a severe breach of faith and conduct.
"Atletico's medical department always respected the privacy of athletes, safeguarding the rights guaranteed by the code of ethics of the profession.
"The club will take the appropriate steps in order to clarify responsibilities in relation to the subject."