Cashner gave up one run on two hits, including a solo homer by Arizona's Ryan Roberts, over 5 1/3 innings in the Cubs' 6-5 win. He had gone to a three-ball count on two batters, including Willie Bloomquist, who drew a walk with one out in the sixth. Cubs athletic trainer Mark O'Neal then went to the mound and, after a brief conversation, Cashner was pulled. The right-hander threw 72 pitches, 43 for strikes.
"He just felt a little tightness," Cubs manager Mike Quade said. "We'll be cautious with everybody, especially early in the season.
"He didn't have any trouble in camp. This caught us off guard completely. He was honest with us, and hopefully we were able to get him out of there before anything bad happened."
The Cubs didn't expect any update on Cashner's shoulder until Wednesday. They were watching his pitch count a little more closely than the other four starters.
He was used in relief last season, appearing in 53 games. Cashner was a starter almost exclusively in the Minor Leagues, compiling a 6-1 record and 2.13 ERA in nine starts between Double-A Tennessee and Triple-A Iowa in 2010.
"The requirements and the things we're asking of him are the same," Quade said. "But we're not asking for 17, 18 pitches to get three outs and adios. I am glad he was here last year and he's had the Wrigley Field experience. I'm glad he's at the back end of this [rotation] and has had a chance to watch his four counterparts first."
Catcher Koyie Hill said he didn't notice anything wrong in Cashner's delivery during Tuesday's game that might signal a problem.
"I didn't even realize anything was going on with him," Hill said. "I noticed the inning before he was doing a really good job separating each pitch, taking his time, really concentrating on his pitch. I remember thinking back a couple pitches, before O'Neal came out there, he was doing something [with his arm] and I didn't think anything of it.
"I think something had grabbed him in his shoulder, and that was it. It's kind of a bittersweet ending to what was turning into a real nice start."
Cashner made it look easy, but Hill said the right-hander is smart enough to know it isn't.
"It was exciting to catch it, and I'm sure it was really exciting for him to be out there throwing it," Hill said. "We were happy with how mature he went about it. He didn't let the situation get bigger than it should've been. He did a nice job. He got ahead, he was really aggressive and came at guys.
"The key for him was that his secondary pitches were strikes as well. He came right at them."
It wasn't just that Cashner had good stuff; he was very poised on the mound.
"The thing you can control is your composure, preparation," Hill said. "I thought he was on top of it today. It was a good start, and he has a long ways to go. I'm sure there will be some downs as much as there are ups in the course of a season. It was a good first step."
Carrie Muskat is a reporter for MLB.com. She writes a blog, Muskat Ramblings, and you can follow her on Twitter@CarrieMuskat. This story was not subject to the approval of Major League Baseball or its clubs.