Darvish, who improved to 7-2 with the Rangers' 12-6 victory over the Blue Jays, allowed seven hits and three runs before leaving. He struck out three and walked three.
Darvish did not have a 1-2-3 inning, and if Toronto hadn't started the game 0 for 8 with runners in scoring position, he might not have lasted those five innings.
"You know he came up with a stiff back and it could've been some of the reason he was erratic," Rangers manager Ron Washington said. "I didn't find out about it until after the fifth inning was over and he was in the dugout.
"He stood out there and gave us five. If he didn't have the stiff back he would've went out for the sixth. At some point I felt he was going to have a quick inning. We weren't taking a chance once he said his back was stiff, so we took him out."
After using a double play to work out of yet another jam in the fifth inning, Darvish said he told the coaching staff he didn't feel good.
"Obviously, I could've gone six and maybe seven, but it's a long season," Darvish said through his interpreter. "I felt that it was the right choice to tell them. In the end the manager made a decision to stop me there."
Darvish said he hasn't felt good physically since his previous start in Seattle, where he lasted just four innings.
"It's more internal, and it has kind of affected my lower back," Darvish said. "I can't say that it's a cold. It's more the stomach area.
It kind of affected the outing today.
"I don't know if it's from travel or going to different climates. If we knew why we got sick we'd never get sick. It's not like I was in pain every pitch I threw. Some of the other non-baseball moves -- bending forward and sitting up there was some uncomfortableness."
Considering their investment in the 25-year-old pitcher, the Rangers intend to be cautious with him.
"I don't want to label it a sore back like I did something to do it," Darvish said. "Like I said, if you have an internal disturbance in the stomach area it tends to affect the body and muscles around it.
"It's not like I'm dealing with an injury. I feel very comfortable saying I should be able to treat this, nurse it and be strong for my next start."
As he left the interview room, Darvish hardly seemed concerned about his back or his stomach.
"We have a dinner event tonight, and as a rookie I'm supposed to wear a wig and sing a song and do certain things that make people laugh," he said. "Maybe my mind was more focused on that than the game."
Texas reliever Alexi Ogando came out of the game in the eight inning after being struck on the right (pitching) hand by a line drive from Brett Lawrie's bat. He's listed as day to day after X-rays were negative.