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03/26/11 1:33 AM ET

Hunter out six weeks or more with groin injury

SURPRISE, Ariz. -- Pitcher Tommy Hunter, who was supposed to be in the Opening Day rotation, will be sidelined for at least six weeks with a strained right groin muscle. The injury once again leaves the Rangers scrambling to fill the back end of their pitching staff.

Hunter injured the muscle during Thursday's outing against the Reds and an MRI showed it to be a Grade 2 or moderate strain. Hunter is scheduled to return to Arlington to receive a blood platelet injection on Monday to help speed the recovery.

He will begin the season on the disabled list and the Rangers estimate he will need at least six weeks of recovery time.

"That's a fair range," assistant general manager Thad Levine said. "We'll have to update that as he responds to treatment and the injection. But that's kind of a ballpark figure."

Hunter's injury occurred mere hours after the Rangers had announced he would be a part of an Opening Day rotation that also includes C.J. Wilson, Colby Lewis, Matt Harrison and Derek Holland. Hunter earned that spot by going 13-4 with a 3.73 ERA in 22 starts and one relief appearance for the Rangers last year. His .765 winning percentage was the best in the American League for a pitcher with at least 15 decisions.

"We had three days to go," Hunter said. "I was looking forward to going back to Texas and being a part of it. Things didn't fall into place."

The Rangers also announced on Thursday that Michael Kirkman had been optioned back to Triple-A Round Rock. Now he is a leading candidate to be in the Rangers rotation although David Bush remains a possibility. Neftali Feliz is not. The Rangers are still committed to using him as a closer.

"We have yet to sit down as a group and see what dominoes will be put into effect," Levine said.

Levine acknowledged that Bush's situation has become more important to the Rangers now that Hunter will miss six weeks. Bush is in camp on a Minor League contract and can take his free agency on Sunday if he is not added to the Opening Day roster. He has not made a decision on that.

"I'd still like to make the team," Bush said. "I don't particularly care, I just want to pitch in whatever role they need. My agent and I have talked about it and what other teams might be interested but I don't have any better idea than that. It all depends on what happens here."

Bush hasn't overwhelmed the Rangers this spring. He has a 4.85 ERA in four outings after giving up four runs in four innings against the Padres on Thursday. But he is a seven-year veteran who has made 184 career starts with a record of 54-67 and a 4.65 ERA. He won 12 games for the Brewers in both 2006-07.

Bush is also a candidate for the final two spots in the bullpen. That has become increasingly competitive now that Mark Lowe is no longer a lock to make the team. Lowe, who has a 14.14 ERA in seven appearances, could be optioned to the Minor Leagues to make room for others.

Pedro Strop, who has a 2.00 ERA in eight games, may have passed him by. The Rangers also like what they see from veteran Brett Tomko, who has allowed just one run over six innings in his last two appearances, and Mason Tobin, the Rule 5 draft pick from the Angels who has a 2.35 ERA in six appearances.

The Rangers have until Opening Day to decide if they want to keep Tobin on the Opening Day roster. Otherwise, as a Rule 5 draft pick, he has to go through waivers and, if he clears, be offered back to the Angels for $25,000. He has missed almost two complete seasons because of serious elbow injuries but has been impressive in camp.

"Mason's been as advertised," general manager Jon Daniels said. "Our scouts did a great job identifying the ability despite limited looks the last couple of years. He's handled himself well in camp and in game situations. Health and consistency are the big keys with him, but he's put himself in the mix for a job with us."

If the Rangers go with Bush in the rotation instead of Kirkman, it allows the Rangers to "keep their depth." That means finding a way to keep as many pitchers as possible and Levine admitted that's an important consideration.

"I think it's real and we experienced it last year," Levine said. "One of the reasons why we won last year was because of our depth."

If Kirkman is added to the rotation and Bush leaves as a free agent, the Rangers are left with a Triple-A rotation of Eric Hurley, Tanner Scheppers and Zach Jackson plus two unknowns. Tomko could also be a part of that if he doesn't win a job in the bullpen.

The Rangers would prefer to have more depth at Triple-A but that took a hit last summer when they included pitchers Blake Beavan, Tanner Roark and Ryan Tatusko in trades. All three would have been candidates to pitch at Triple-A this year.

Brandon Webb and Scott Feldman are expected to begin the season on the disabled list and won't be available until mid-May at least. Now Hunter is on that list. The Rangers went into Spring Training exulting their pitching depth and leave trying to stretch it as far as possible.

T.R. Sullivan is a reporter for MLB.com. Read his blog, Postcards from Elysian Fields and follow him on Twitter @Sullivan_Ranger. This story was not subject to the approval of Major League Baseball or its clubs.