"It's something we've been dealing with the last couple weeks," Angels manager Mike Scioscia said, Shields having concluded after a May 26 effort against the White Sox that he no longer could pitch through the pain on his landing leg.
"Scot's a guy, we've seen what he's done in our 'pen for a number of years. To lose a guy like Scot, we'll have to have a few guys step up and fill the role."
Shields said he didn't know exactly what type of procedure will be performed by Dr. Lewis Yocum, the team orthopedist.
"I wanted to keep playing, but when the pain won't go away, it's time to get it fixed," Shields said. "When you're more worried about the pain in your leg than focused on the hitter, you know it's something you have to take care of, and that was the case.
"It's wear and tear, not any one [incident]. It's something I feel I have to do to come back 100 percent next spring. The way I was pitching wasn't fair to me or to the team."
Shields has dealt with pain in the knee to varying degrees since last season, but it didn't stop him from having another productive season. He came into 2009 having thrown more innings (425) over the past five seasons than any reliever in the Majors.
He has led the American League in holds with 31 each of the past three seasons. Now that vital eighth-inning role will be handled by a committee that includes Darren Oliver, Jason Bulger, Kevin Jepsen and Justin Speier.
Scioscia said general manager Tony Reagins "is talking every day to other clubs" about possible deals, but pitching is thin almost everywhere, making a significant deal difficult without overpaying.
Another candidate, Kelvim Escobar, hopes to join the mix soon. Escobar was removed from the rotation when he discovered he couldn't get past 75 pitches without experiencing pain in his right shoulder, subjected last July to surgery to repair a labrum tear.
Escobar, with considerable relief experience both in Anaheim and Toronto, would be the best answer if his arm responds when he begins playing catch on Monday in San Francisco.
"We have guys who've done a good job," Escobar said, identifying Jose Arredondo (now in Triple-A Salt Lake after a rough start) along with Bulger, Oliver, Speier and Jepsen. "Going to the bullpen, it's nothing new to me. I'm comfortable there. It would be good for me and the team if I'm able to set up."
The bullpen's collective ERA of 5.84 is the worst in the AL. Shields is 1-3 with a 6.62 ERA, working 17 2/3 innings in 20 appearances.
He made a return appearance this spring for Team USA in the World Baseball Classic but struggled to find his form from the start of the season.
A stand-up guy who never searched for excuses, he resisted talking about the knee as a hindrance until it was time to go on the 15-day disabled list on May 27.
Lyle Spencer is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.