Baltimore placed Hammel on the 15-day disabled list Sunday.
“I can’t keep pitching on a bad leg,” he said. “I start feeling things with my arm and hurting stuff there. I’m obviously bummed out.”
Hammel’s knee has been bothering him since May, but became more serious Friday night when he was forced to leave a game against Detroit after throwing a pitch in the fourth inning.
Hammel had an MRI on Saturday that revealed loose cartilage, and that persuaded him to have the operation.
“It’s a decision that had to be made,” he said. “It’s not easy because I could rest for maybe a week and it might go back to where it was. Then you run the risk of it happening every time I go out.”
Obtained in a December trade with Colorado, Hammel (8-6, 3.54 ERA) leads Baltimore in wins and strikeouts (106).
To fill Hammel’s spot on the roster, the Orioles recalled reliever Steve Johnson from Norfolk.
YOUKILIS KEEPS QUIET
Chicago White Sox third baseman Kevin Youkilis refused to comment Sunday on the latest criticism from Boston Red Sox manager Bobby Valentine.
Valentine held Youkilis responsible for the rift between the two.
“I got nothing about any of that stuff,” Youkilis said before a game against the Kansas City Royals. “I’m over all the Boston thing of this year. There is a lot of great past history, but the focus is on the White Sox.”
The Red Sox traded Youkilis on June 24 to the White Sox, where he entered Sunday hitting .316 with 15 RBI in 15 games. The White Sox open a three-game series Monday at Boston.
On April 15, Valentine said: “I don’t think he’s as physically or emotionally into the game as he has been in the past for some reason.”
“I think the comment I made early, he made a big issue out of, and I don’t think he ever wanted to get over it,” Valentine said Sunday.
SHORT HOPS
Phillies ace Roy Halladay is scheduled to be activated from the 15-day disabled list on Tuesday and start the game that night against the Los Angeles Dodgers. … Angels reliever Jordan Walden was placed on the 15-day disabled list with a strained biceps. … The Reds are unsure whether right-hander Johnny Cueto will be able to start as planned Tuesday because of a blister on the index finger of his pitching hand. Cueto was originally supposed to start against St. Louis on Sunday. … Blue Jays pitcher Sergio Santos will miss the rest of the season with a shoulder injury. Santos has been on the disabled list since April 21. … Dodgers right-hander Chad Billingsley was scratched from Sunday’s start against the San Diego Padres because of a sore elbow. He underwent an MRI exam Sunday and left Dodger Stadium without talking to reporters. … Mets right fielder Lucas Duda will have his left hamstring examined by a doctor in New York today. ... The Astros placed infielder Jed Lowrie and catcher Jason Castro on the disabled list.

