Duchscherer, who had a Major League-best opponents' batting average of .208 and the second-best ERA (2.59) in the American League entering Monday's game, had thrown 2 2/3 shutout innings when he stepped off the mound after striking out Nick Punto in the bottom of the third. Duchscherer summoned A's manager Bob Geren and head athletic trainer Steve Sayles while slightly shaking his right leg.
After a brief conversation, Duchscherer (10-8) was escorted off the field and replaced by righty Kirk Saarloos.
"I'm just trying to stay positive," Duchscherer said. "They think I might just have irritation in the joint. Hopefully that's all it is."
The A's are sending Duchscherer back to the Bay Area, where he'll be examined by team orthopedist John Frazier on Tuesday.
"I'm very concerned," Geren said. "It's obviously a big concern. He's our ace and our All-Star."
Duchscherer, who suffered a strained right hip in May 2007 and had season-ending surgery in early July, worked exclusively out of the A's bullpen from 2004-2007 but was moved into the starting rotation this spring in hopes that a more predictable workload would keep him healthier.
He suffered a strained right biceps in his first start of the season, however, and spent three weeks on the disabled list.
If Duchscherer's most recent injury lands him on the DL, it will mark the 22nd time the A's have used the list, tying the Oakland record set in 1992 and matched last season.
Mychael Urban is a national writer for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.