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06/20/12 7:58 PM ET

Rangers shake up their pitching staff a bit

SAN DIEGO -- The Rangers' pitching staff will have a slightly new look heading into this weekend's series with the Rockies.

The big news was the callup of veteran righty Roy Oswalt from Triple-A Round Rock to a starting rotation hampered by injuries to the likes of Derek Holland and Neftali Feliz. Oswalt will start on Friday, and will assume the spot vacated by Tuesday's winner Scott Feldman, who will move back into the Rangers' bullpen.

Feldman was 0-6 with a 7.36 ERA over six starts heading into Tuesday's outing, but threw six innings of two-run ball to earn a win. Feldman will assume the role of long reliever, and his return to the 'pen restores the Rangers' relief staff to seven arms -- manager Ron Washington's preferred number.

"It's an advantage just to be back to seven relievers now," Washington said. "So far we've been very, very fortunate that we've been able to come out of the bullpen with the pieces we want to come out with. That's because our starters have been doing their job."

Washington also announced that rookie Justin Grimm will remain in the starting rotation and start on Monday, nine days after he won in his Major League debut against the Astros last Saturday. The 23-year-old was called up from Double-A Frisco, where he went 8-3 with a 1.87 ERA.

"What I liked more than anything else was his mound presence," Washington said of Grimm. "He showed that he can command the baseball, he has an idea of how to pitch. ... The kid got us into the sixth inning, he did a good job, so he deserves to get the ball again."

Injured Feliz, Holland throw on Wednesday

SAN DIEGO -- Injured Rangers starters Neftali Feliz and Derek Holland continue to progress in their rehab, as both threw before Texas' series finale in San Diego on Wednesday afternoon.

Feliz has not pitched since May 18 due to inflammation in his right elbow, and was placed on the disabled list on May 21. He has since been moved to the 60-day DL, and is eligible to return on July 18.

The 24-year-old righty, who made a name for himself in the Rangers' bullpen over the past few seasons, threw for the first time since the injury on Monday, and put in a similar session before Wednesday's game, throwing lightly from 60 feet just to, as manager Ron Washington put it, "get his arm going." Feliz is 3-1 with a 3.16 ERA and one complete game in seven starts this season.

Holland made a bit more of a jump in his rehab on Wednesday, throwing his first bullpen session since being placed on the disabled list on June 7 with shoulder weakness resulting from a stomach virus. The 25-year-old also had successful long-toss sessions earlier in the week.

The lefty's rehab can progress quickly because it was not so much injury but illness and the accompanying weight loss that sidelined him in the first place. Holland is 5-4 with a 5.10 ERA in 12 games, 11 starts, this season.

Stint on 15-day DL likely for Moreland

SAN DIEGO -- Rangers general manager Jon Daniels said he expects first baseman Mitch Moreland, who left Tuesday night's game with a strained left hamstring, to land on the 15-day disabled list.

Moreland had an MRI on Wednesday morning, but the team will wait to decide his prognosis until the club returns to Arlington on Wednesday night. The 26-year-old Moreland tweaked the hamstring stretching for first while running out a ground ball Tuesday night.

"It's not too often a guy comes off the field holding his hamstring and it isn't at least two weeks," Daniels said. "But we'll make sure. We'll get the MRI read, he'll see the doctor tomorrow, and we'll make the call then."

Daniels said the decision will most likely be announced on Friday.

Chelsea Janes is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.