Darvish, who has been sidelined by triceps and elbow injuries, shook his arm as he left the field after striking out Great Lakes' Romer Cuadrado with a 95 mph fastball for the last out of the first. But he said the first inning was great.
"I felt real good,'' Darvish said through an interpreter. "I didn't feel any abnormalities in that sense.''
When Darvish warmed up for the second, he threw six pitches and winced on each one. Miguel Amaya, who caught Darvish's rehab start for South Bend on June 25, walked to the mound and summoned manager Jimmy Gonzalez and a trainer. After a short discussion, Darvish walked to the dugout.
"During warm-ups the next inning, I felt something in there,'' Darvish said. "Last time [in June] I did feel the same thing, [but] I continued to throw. This time I stopped.''
Darvish, who turned 32 on Thursday, signed a $126 million, six-year contract with Chicago in February. He has made just eight starts in his first year with the Cubs, going 1-3 with a 4.95 ERA.
Darvish said he would ask for an MRI when he got back to Chicago. He wasn't sure if he would miss the rest of the season.
"I can't really tell right now,'' he said. "It depends how I feel tomorrow. I really want to come back this season. I am going to work hard and do my best to try and come back.''
The Cubs are an NL-best 71-52, despite a 2-1, 11-inning loss to the Pirates on Sunday, but their rotation is looking a little thin. Mike Montgomery was placed on the 10-day disabled list on Friday with left shoulder inflammation, and Tyler Chatwood has struggled in his first year with Chicago after signing a $38 million, three-year contract over the winter.
The Cubs were optimistic about a possible return for Darvish after he threw about 55 pitches in a simulated game on Tuesday. But that optimism was likely wiped out by Sunday's rehab appearance.
"I was looking for it. I can't deny that," Cubs manager Joe Maddon said. "I've been looking forward to getting him back, have been all year, but ... you have to go out there with the guys you have."
Darvish allowed a first-pitch single and issued a walk to his first two batters, and then he retired three in a row. He totaled 19 pitches, 10 for strikes.
"It's unfortunate," Cubs teammate Anthony Rizzo said. "He's working hard to get back. For that to happen, it stinks for him."
In his June rehab start for South Bend, Darvish allowed one run and three hits in five innings. His fastball reached 94 mph, but he complained afterward that his arm got tired as the outing went on.
ESPN's Jesse Rogers and The Associated Press contributed to this report.