Flanagan missed much of training camp because of patellar tendinitis in his left knee. He started the first three games of the season but was able to finish only one.
Journeyman offensive lineman Grey Ruegamer, who replaced Flanagan in the two games the ninth-year veteran could not complete, now moves into the starting lineup. Ruegamer, a solid player who had only three career starts in four previous seasons, is well-schooled in Green Bay's offense. He recently agreed to a pay cut to help the team's salary cap situation.
"Mike has experienced tremendous pain with this, has played with it, and it's just been debilitating," said coach Mike Sherman of the tendinitis in Flanagan's left patellar tendon. "He's going to have to have surgery in the next 10 days and it will probably take about six months for recovery."
Earlier this season, Flanagan, 30, was described by one Green Bay coach as the "most valuable lineman" on the Packers' roster. An extremely versatile player, he has moved out to play tackle in the past, when injuries precipitated a lineup shuffle, and is one of the game's best technicians.
The former UCLA star joined the Packers as a third-round pick in 1996. He struggled with injuries earlier in his career, appearing in only two games in his first three seasons before settling into the starting lineup in 2001.
Flanagan is so key to the Packers, whose line unit has been together for four seasons, that Sherman often limited Flanagan's practices between games, so he would be available on Sunday afternoons.
Green Bay will place Flanagan, who has appeared in 84 games with 54 starts, on injured reserve. His roster spot probably will be filled by Scott Wells, a rookie offensive lineman and seventh-round draft pick who has been on the Packers' practice squad.
Flanagan first experienced pain in his left knee last season but attributed it to the normal wear and tear associated with offensive line play. Doctors then diagnosed the tendinitis condition during the spring.
He had hoped to avoid surgery, which he knew was inevitable, until after this season, but the pain became insurmountable in recent days.
"There's nothing left to do," Flanagan told the Green Bay media of his condition. "We've exhausted all (other) attempts. Part of my tendon is not getting blood flow, and so it can't recover. That's what tendinitis is. It's one of those things."
Len Pasquarelli is a senior writer for ESPN.com. To check out Len's chat archive, click here.