Shields had the operation Tuesday in Los Angeles to repair the patella tendon in his knee and the Angels transferred him to the 60-day disabled list. Team orthopedist Dr. Lewis Yocum performed the surgery.
The team expects Shields to resume baseball activities in about six months.
Shields, who turns 34 next month, went 6-4 with a 2.70 ERA last season, but was 1-3 with a 6.62 ERA in 20 appearances this year. He gave up runs in eight of those outings, and had three losses and three blown saves before going on the DL. His 42 wins as a reliever are the most in franchise history.