"It appears as though the rehab could take 8 to 10 months," a person familiar with the injury told The Associated Press on condition of anonymity because the team has not made an official announcement.
"For all intents and purposes, his career is over," the person, who confirmed the nature of the injury, said in an e-mail Tuesday.
The Patriots officially placed Harrison on the injured reserve list Wednesday, ending his season.
Harrison, a 15-year veteran, two-time Pro Bowler and two-time Super Bowl champion, tore the quadriceps muscle without being hit while chasing scrambling quarterback Jay Cutler on the last play of the third quarter of the Patriots' 41-7 win over the Denver Broncos on Monday night.
Harrison has had persistent thigh problems as far back as last season's playoffs.
"He's had a slight tear all [this] season," Bill McLaughlin, a marketing representative for Harrison, said Tuesday. "He was getting treatment on it."
Harrison was listed on the Patriots' injury report with a thigh problem before their first two playoff games last season. The injury kept him out of practice two days before New England's 21-12 win over San Diego in the AFC Championship Game. He was not on the injury report for the Super Bowl, won by the New York Giants 17-14.
Harrison played in all three of those games.
The NFL Network first reported Tuesday that Harrison had a torn quad. The Boston Globe later reported he had a torn right quad. During the game, it was announced he had a knee injury.
Harrison played in all six games this season and was not listed on any injury reports but "was injured the whole time," McLaughlin said. "He's played through injuries before."
Harrison, in the final year of his contract, might have retired anyway after this season following a succession of serious injuries in two of the three previous years.
After being injured Monday, Harrison pointed to his teammates and waved to the crowd as he was driven off the field on a cart.
Coach Bill Belichick gave no update on the injury during his conference call Tuesday, but said after the game that it didn't look good.
"It was difficult for all of us to watch Rodney be carted off like he did," Belichick said Tuesday. "We hope that all goes well for him."
The Patriots (4-2) already are without quarterback Tom Brady, who suffered a season-ending knee injury in the opener, and running back Laurence Maroney, whose season ended when he went on injured reserve Monday with a shoulder injury. Belichick refused to say if Maroney would need surgery.
Sammy Morris rushed for 138 yards in his place but hurt his knee and didn't play in the second half. Belichick said his status was day to day.
The injury to the 35-year-old Harrison is his fourth in four years.
In 2005, he tore three ligaments in his left knee in the third game and missed the rest of the season and the playoffs. The next season, he sat out six games with a broken right shoulder blade and returned for two before suffering a strained right knee in the final regular-season game and missing the playoffs.
He was healthy last season but missed the first four games for violating the league's substance-abuse policy. He said he used the substance, believed to be human growth hormone, to speed up his healing process.
This season, he started all six games and had one interception.
Harrison made five tackles Monday before the injury.
"Rodney is one of the leaders on the defense from experience and from his playing style and production," Belichick said. "He's a good player. He's been a good player [for the Patriots] just going on six years now.
"It was hard to watch him go through what happened last night. That's tough. I feel badly for him. You hate to see that with any player, but that was very unfortunate for him and we all feel badly for him."
Safety Brandon Meriweather, a first-round draft choice last year, got his third interception of the season Monday and likely would move into a starting role.
Harrison made the Pro Bowl in 1998 and 2001 during his nine-year stint with San Diego. The Chargers cut him after the 2002 season when he was slowed by an ankle injury.
The Patriots signed him as a free agent in March 2003 and they won Super Bowls in each of their first two seasons with him. In six seasons with New England, he had nine sacks and eight interceptions.
The aggressive Harrison, the target of complaints by some opponents of dirty hits, has been fined more than $200,000 by the NFL. He was suspended for one game in 2002 for a helmet-to-helmet hit on Oakland's Jerry Rice that cost him a game check of $111,764.
But Harrison also has been a mentor to players like 24-year-old James Sanders, the Patriots' other starting safety.
"He is a future Hall of Famer. He is one of the best to play this game. He has taught me a lot," Sanders said in the locker room after Monday's victory. "I am going to go to the training room and see how he is doing and let him know I am here for him."
Information from The Associated Press was used in this report.