The complaint, which is the third of that kind that the Argentine has suffered in as many years, is expected to sideline him for up to nine months.
"Ale has had his ACL reconstructed, using a different graft. He also had a very little reinforcement, which is a slightly different operation to the one he had previously," QPR's head of medical services Peter Florida James told the club's official website.
"The surgeon was very pleased with how things went which is positive. Ale is still in hospital, but he's in better spirits now than he was a week or so ago, so we're all grateful for that."
The 28-year-old signed on at Loftus Road back in 2009.