WASHINGTON - Washington Redskins quarterback Robert Griffin III has a torn lateral collateral ligament in his right knee and will undergo surgery in coming days, a person familiar with Griffin's condition said Tuesday night.

Damage to that ligament could mean a lengthy rehabilitation that might jeopardize the star rookie quarterback's chances of being ready for the start of the 2013 season in September and could keep him sidelined even longer, according to two experts.

When they perform the surgery, doctors also will determine whether a second ligament, Griffin's anterior cruciate, has been damaged as well, according to the person familiar with Griffin's case, who spoke on the condition of anonymity because he was not authorized to discuss the matter publicly.

Griffin was examined Tuesday outside Pensacola, Fla., by orthopedic surgeon James Andrews. The Washington Post reported Monday that an initial MRI exam of Griffin's knee showed possible tears of the two ligaments. Coach Mike Shanahan said at a news conference Monday that the test was inconclusive and that Griffin would see Andrews to determine whether those results reflected new injuries or previous ones.

James Dreese, a doctor for University of Maryland athletic teams, said an LCL tear requires a longer rehabilitation process than an ACL tear. Dreese, who has no specific knowledge of Griffin's injury, said an LCL tear could keep Griffin out for eight to 12 months.

The length of the rehabilitation depends on whether the ligament can be repaired surgically or requires a more extensive reconstruction using a graft from another part of the body or a cadaver. An ACL typically requires a six- to eight-month rehabilitation.

"When the collateral ligaments are involved," Dreese said, "the concern in the long term is that controlling the rotational component of the knee can be more difficult."

Dreese also said LCL tears are usually accompanied by injuries to one of the cruciate ligaments - the ACL or PCL (posterior cruciate ligament). However, he said if there is an ACL tear, in addition to the LCL tear, it would not necessarily lengthen Griffin's rehab.

Griffin tore the ACL in the same knee in 2009, when he was a sophomore at Baylor University.

Tim Kremchek, the Cincinnati Reds medical director and chief orthopedic surgeon, also said that a full-scale LCL tear could be more devastating than a torn ACL. Unlike the ACL, the LCL cannot be surgically repaired with an arthroscopic operation, he said.

"It could be worse," Kremchek said Monday, before word of Griffin's condition emerged. "It depends on how much is done. It's a big, open incision. The rehabilitation is slowed down. It's not as quick to make the muscles strong. It's dicey. The outside ligaments are a worse problem. They're not as predictable as an ACL."

Minnesota Vikings running back Adrian Peterson, who tore his ACL on Dec. 24, 2011, in a game against the Redskins at FedEx Field in Landover, defied the odds and returned to action by the start of the 2012 season. He played in all 16 games, finishing nine yards shy of the NFL's single-season rushing record.

Griffin initally sprained the LCL four weeks ago when he was hit in a win over the Baltimore Ravens. He appeared to reinjure it in the first quarter of the Redskins' 24-14 playoff loss to the Seattle Seahawks on Sunday, when he planted his foot as he tried to throw a pass across his body. He remained in the game until the fourth quarter, when his knee buckled awkwardly and he could not continue.

Since then, controversy has swirled throughout professional football about whether Shanahan should have pulled his star rookie from the game despite Griffin's insistence on continuing to play even as his mobility appeared to diminish.

DeMaurice Smith, executive director of the NFL Players Association, said Tuesday that the union is looking into the Redskins' handling of the quarterback's injury. Smith said the union also is looking into the condition of the playing surface at FedEx Field during Sunday's game, which was criticized by, among others, Seattle Coach Pete Carroll.

Griffin and team owner Daniel Snyder arrived at the Andrews Institute for Orthopaedics and Sports Medicine in Gulf Breeze, Fla., on Tuesday. The appointment was believed to have lasted just more than two hours, according to a person with knowledge of the situation. But it wasn't clear what kinds of tests were conducted.

The Redskins had no comment about the results of Griffin's examination. The team has said it would make a formal announcement "at the appropriate time," and when there is a definitive diagnosis about Griffin's injury.

On Dec. 10, a day after the Baltimore game, Shanahan announced that MRI exam findings revealed that "Robert has a mild strain of the LCL. It's Grade 1. He's had some mild swelling. He had his right knee repaired - his ACL - in 2009. That looks great. No problem there."

In the first quarter Sunday, however, Griffin came up hobbling badly after rolling to his right, planting on the injured right leg and throwing an incomplete pass. Griffin limped back to the huddle and two plays later threw a touchdown pass, but his mobility decreased significantly over the next 2 1/2 quarters.

Midway through the fourth quarter, one play after he was sacked for a 12-yard loss, Griffin's knee bent awkwardly as he scrambled for a loose ball. Team trainers and Andrews, who is on the sideline for every Redskins game, tended to him. Griffin was helped up, limped off the field and did not return.