"It's about trying to get Harry right. We can't have him going out there and he's having issues," manager Ron Washington said. "We're going to get those issues taken care of by giving him time down, get it right, and when he comes back we've got him for the rest of the year."
Justin Grimm, who was a candidate for the No. 5 spot in spring training, will be recalled from Triple-A Round Rock and start in Harrison's spot against the Mainers.
Harrison (0-2, 8.44 ERA) has been dealing with stiffness in his back causing weakness in his left leg since late in spring training.
The DL move is retroactive to Saturday, the last day he pitched. He gave up a pair of two-run home runs in the first inning and allowed five runs in five innings against the Los Angeles Angels.
Harrison had an epidural injection Sunday, and said his back has felt better each day. Even though he didn't throw his scheduled bullpen session Tuesday, Harrison is still able to play catch and keep his arm loose.
"It's better to miss two starts than miss 15 or 20 because I made it worse," Harrison said. "Hopefully this shot's starting to work, get that inflammation off that nerve, and that way I can just maintain it from there and manage it."
The 27-year-old Harrison, in the first year of a $55 million, five-year contract, has never had any back issues before. He was an 18-game winner and an All-Star last season.
"I've got to get this time to heal up and get better so I don't further any injury or overcompensate anywhere else and end up having arm issues," Harrison said.
Yu Darvish, who has been bothered by a blister on the ring finger of his right hand, is scheduled to pitch Friday in the second game of the series against the Mariners.
Darvish (2-0, 1.98) said Wednesday that he felt fine and that all was good in his bullpen session.
"I was able to command my pitches and I was able to throw most of my pitches," Darvish said through his translator. "I think it went well."
Asked if he was going to have to make any adjustments, Darvish said he wouldn't.
"Everything is still good, still on schedule," Washington said of Darvish. "I looked at his hand, (the blister) is even smaller. It's amazing how skin heals. So just hope he doesn't have any issues with it."
Grimm won his Triple-A opener, allowing one run and four hits over 5 2-3 innings. He made his major league debut with the Rangers last season, when he went 1-1 in five games (two starts) and allowed 14 earned runs in 14 innings.
"I just expect him to come and keep us in a ball game, that's it," Washington said. "He's a strike thrower and I think he understands how he's supposed to use those strikes. And if he come up here and does that, I think he'll be fine."