It was initially thought that Milito will be out of action for six months after sustaining an injury in Barcelona's Champions League clash against Manchester United last month.
However, surgeon Ramon Cugat revealed that the 27-year-old is set for a prolonged period on the sidelines.
"normally, a cruciate ligament injury takes about six months to recover from,"Cugat was quoted as saying by Barcelona's official website.
"In this case, we were talking about a replacement cruciate ligament because it already underwent surgery seven years ago in Argentina.
"the six weeks may end up being as many as nine, as operating on the same knee, and taking out one plasty and replacing it with another, can lead to added complications."
Milito joined Barcelona from Real Zaragoza in July 2007 but struggled to impress in the heart of the team's shaky defense.