Orthopedist James Andrews said Wednesday that Griffin "had a direct repair" of the lateral collateral ligament and "a re-do of his previous ACL reconstruction."
Andrews says he expects a full recovery because of Griffin's "high motivation."
Griffin reinjured his knee in Sunday's playoff loss to the Seattle Seahawks. He previously tore the ACL in the same knee while playing for Baylor in 2009.
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RG3's injury could lead to changes on sidelines
Now comes RG3's next challenge: a race against time to get healthy before the beginning of next season.
Griffin's recovery time will take several months at least but could extend into next season, depending on the extent of the damage. The process could also be speeded up because Griffin is known as a focused, determined competitor who would be expected to take his rehab as seriously as possible.
"Thank you for your prayers and support. I love God, my family, my team, the fans, & I love this game. See you guys next season," Griffin tweeted before the surgery began.
Athletes generally need nine to 12 months to make a full recovery from a torn ACL, although Minnesota Vikings running back Adrian Peterson made a remarkable return this season about eight months after tearing an ACL -- and nearly broke the NFL's single-season rushing record.
Russ Paine, a physical therapist in Houston who worked with Peterson during his rehab, said the running back's timetable for returning to competition was in fact "pretty traditional."
"What's non-traditional is him almost breaking the rushing record," Paine said in a phone interview Wednesday.
Griffin reinjured his knee at least twice in Sunday's playoff loss to the Seattle Seahawks, prompting a national debate over whether coach Mike Shanahan endangered Griffin's career by not taking the team's franchise player out of the game sooner.
The first major injury to the knee came in 2009, when Griffin tore the ACL in the third game of the season while playing for Baylor. Griffin missed the rest of the year but returned in 2010 and won the Heisman Trophy in 2011.
Griffin sprained the LCL last month when he was hit by Baltimore Ravens defensive tackle Haloti Ngata at the end of a 13-yard scramble. Griffin missed one game and returned to play three more while wearing a bulky knee brace, his mobility clearly hindered.
Then, on Sunday, Griffin hurt the knee again as he fell awkwardly while throwing a pass in the first quarter against the Seahawks. He remained in the game, with Shanahan saying he trusted Griffin's word that all was OK.
Griffin finally departed in the fourth quarter, after the knee buckled while he was trying to field a bad shotgun snap.
The No. 2 overall pick in last year's draft, Griffin was one of several rookie quarterbacks to make an instant impact on the NFL this season. He set the league record for best season passer rating by a rookie QB and led the Redskins to their first NFC East title in 13 years.
But he also had to leave three games early due to injuries -- two because of his knee and one because of a concussion -- and missed a fourth altogether because of the knee.