"Right when I landed [on the second pitch]," Hammel said of the pain. "I've always been a stiff front leg guy landing, and I felt a sharp pain in the back of the knee. MRI tomorrow, and we'll figure out exactly what it is."
Hammel said the pain was enough that he shut himself down.
"Every pitch afterword got tighter and tighter and more painful," he said.
Hammel did retire the side in order in the first, and pulled up when Matt Carpenter hit a grounder to first baseman Anthony Rizzo for the second out.
Before Jorge Soler's at-bat in the Cubs' first, Chicago manager Joe Maddon gave home-plate umpire Pat Hoberg a heads up that Hammel was injured. Clayton Richard took over in the second.
This was Hammel's 17th start of the season, and not how he wanted to end the first half.
"It's very frustrating," Hammel said. "Tomorrow morning, hopefully, I can find out some good news. Hopefully, it's just a strain or something like that and easily treatable and the silver lining is it's the All-Star break. It's probably the best timing for it to happen."
Carrie Muskat is a reporter for MLB.com. She writes a blog, Muskat Ramblings. You can follow her on Twitter @CarrieMuskat and listen to her podcast. This story was not subject to the approval of Major League Baseball or its clubs.