"A scan carried out this morning confirms that he has a torn muscle which will keep him off the pitch for a period of about six weeks," it said.
The former FIFA world player of the year has missed Barcelona's last four Spanish league matches and only returned to training with the rest of his team-mates on Wednesday.
Barcelona coach Frank Rijkaard said the player had been absent because he had complained of muscle pains in his right leg.
But a report from the Catalan club's medical staff, published on March 17 on Barcelona's website, said scans found he had "neither a tendon nor muscular injury".
Spanish media has reported that Rijkaard has grown tired of the player's frequent late nights out and was not counting on him for the rest of the season.
Ronaldinho, who is under contract with Barcelona until 2010, has had his worst season since he joined the Catalan side from French side Paris Saint-Germain in 2003.
Italian newspaper Gazzetta dello Sport published photos on Thursday of his brother and agent Roberto Assis at a Milan restaurant with Milan vice-president Adriano Galliani which fuelled speculation that the player will join the Italian club.
And Milan coach Carlo Ancelotti has confirmed the club are keeping tabs on Ronaldinho.
"Milan have been interested in this player for a long time. Even if he stays at Barcelona the club will assess the situation," Ancelotti said.
"Ronaldinho is a great champion, and therefore unfortunately he is a player that not only interests us," he added.
Premier League side Chelsea has also been touted as a possible destination for the player at the end of the season.
Barcelona lie in third place in the Spanish league with 58 points, seven behind defending champions and leaders Real Madrid.
Without Ronaldinho, Barcelona on Tuesday defeated Schalke 1-0 in the first leg of the Champions League quarter-finals.
Ronaldinho helped Barcelona win the Champions League in 2006 and the Spanish league title in 2005 and 2006.