Cuban Odrisamer Despaigne was added to the 25-man roster from Triple-A El Paso to make the start.
"I've been dealing with a little bit of shoulder soreness. We really don't know where it's from. But I think it's best to step back, get healthy and come back and help us win," Cashner said.
Cashner said he first felt the soreness after a start on the road against the Mets on June 13. But five days later, he allowed one run over seven innings against the Mariners.
"After I came out of the game in New York, it was really tight across my shoulder," Cashner said. "I never really recovered from it."
Cashner, who missed most of the 2011 season with the Cubs due to a strained rotator cuff, is confident he'll be back on the mound soon.
"I hope to be playing catch by the end of the week. So hopefully we can knock this out," he said. "The previous shoulder injury I've had, I've felt where there was something wrong."
It's been a difficult stretch for Cashner, who missed 20 games with elbow soreness, returned to make three starts and then landed on the disabled list yet again.
"It's been tough. I feel like I've had a good feel this whole year for my stuff, had a good mindset of what I've wanted to do. It's definitely been tough. But I didn't feel like I could keep going the way I was feeling."
Corey Brock is a reporter for MLB.com. Keep track of @FollowThePadres on Twitter. This story was not subject to the approval of Major League Baseball or its clubs.