Affeldt was placed on the 15-day disabled list Sunday, while San Francisco recalled right-hander Jean Machi from Triple-A Fresno to take his place.
Affeldt, who already missed time with a strained right oblique muscle, said he had experienced some discomfort in the groin for a couple of weeks and might have "cost" the team some games by pitching through it because he altered his delivery and pitch selection. He said it's possible the two injuries are related.
"Sometimes being a tough guy doesn't always work out too well," Affeldt said. "Sometimes you've got to learn when not to be one."
Manager Bruce Bochy said an MRI exam Saturday night showed a moderate groin strain. Jose Mijares will now take on a bigger role out of the bullpen.
"Jeremy's going to be out a while," Bochy said. "That's a tough area for a pitcher. It takes a while to heal."
Affeldt is 1-4 with a 3.51 ERA in 38 appearances and 33 1-3 innings this season, but 0-3 over his last 15 outings with three blown saves -- and has blown all four of his total save chances. Bochy had spoken to Affeldt before the injury about backing him off until he got back on track.
Bochy said if Affeldt was hurting while on the mound, he didn't tell anybody, saying, "If that's the case, he kept that to himself."
"As far as I know he's been good to go. It hasn't played a part in whether I was going to use him or not," Bochy said. "It's too bad this happened because he's an important part of the thing."
The 34-year-old Affeldt has had his share of rough injury luck in recent seasons -- much of it sustained in off-the-field mishaps.
In April 2011, he sprained his right knee and went on the DL after reaching out to catch his then-4-year-old, 60-pound son when the boy jumped off the couch to greet his father. On Sept. 8 that year he sliced his non-throwing hand nearly to an artery separating frozen hamburgers during an outdoor barbecue with his family on an off-day. The paring knife he was using pushed through a hamburger patty and deep into his hand. Affeldt came within a millimeter of an artery and underwent surgery about eight hours after the injury to repair nerve damage in his pinkie.
"I got hurt on the field, which is comforting to me," Affeldt said Sunday.
After going 1-2 with a 2.70 ERA in 67 appearances over 63 1-3 innings last season for the World Series champion Giants, Affeldt received an $18 million, three-year contract. Re-signing the reliable reliever was among general manager Brian Sabean's top priorities.