The Mariners switch-hitting outfielder, sidelined since July 27, had the lateral meniscus tear repaired in Cincinnati by Dr. Timothy Kremchek.
"He'll start his rehab tomorrow and will be out four to six weeks," Mariners interim manager Daren Brown said. "Doing the math, it's real close to [the end of the season].
"If it's on the four-week, we'll see. But I would say it looks like (he could be out for the season)."
It has not been the season either the Mariners or Bradley expected.
Acquired from the Cubs on Dec. 18 for right-handed pitcher Carlos Silva, Bradley played in 73 games for the Mariners, batting .205 (50-for-244) with eight home runs and 29 RBIs.
Besides the time he spent on the disabled list, the mercurial outfielder missed part of season to address his "emotional stress" from personal issues.
He was removed from the Mariners' game on May 5 by then-manager Don Wakamatsu after striking out twice against the Rays. An upset Bradley left Safeco Field immediately and returned the following day, asking the organization for help dealing with his emotions.
Earlier in the season, Bradley had said the Mariners were a perfect fit for him.
How important would it be for Bradley to play a few more games this season?
"I would like to think it would be important," Brown said, "but at the same time, when you are doing something like that this late in the year, health is more important. If it takes more time for him to be healthy, that's fine, too."
Bradley has one year remaining in the three-year, $30 million contract he signed with the Cubs prior to the 2009 season. The Mariners are obligated to pay him the remaining $12 million next year.
Jim Street is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.