The 25-year-old righty reported after the game he felt the pain on his second and third warmup pitches, prompting him to signal to the dugout for Padres manager Bud Black to come out with the trainers.
"I felt a little grab and then the third pitch, it grabbed even worse," Cashner said. "It's definitely sore right now."
Cashner allowed one hit and struck out two in his two innings of work.
The latissimus dorsi muscle is located below the shoulder on the side of the torso. Padres closer Huston Street missed a month earlier this season with a similar injury.
"Our diagnosis from our medical staff is outstanding," Black said. "We have a lat strain of some sort to some degree. Pretty low on the lat, not the shoulder though."
Black said he was pleased that Cashner didn't attempt to pitch through the injury and cause more damage.
"He felt it and he did the right thing," Black said. "He felt like he could keep going, but he said he wanted to let us know what was going on."
Tyler Emerick is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.