"We're pretty confident that he will not start with us," Royals manager Buddy Bell said. "He probably won't throw at the earliest for 10 days, so he'll probably start [the season] on the disabled list."
Bell said it was similar to the injury, which is in his back and under the shoulder, which Mike MacDougal suffered last March that kept him out until June.
"It is not as severe," Bell said. "We don't think it will be that long. It's a strain and MacDougal's was a tear."
The Royals optioned left-hander Neal Musser to Triple-A Omaha on Thursday. Bell said he doesn't "feel a need" to have two left-handers in the 'pen.
"We would like to have two left-handers, but we're going to take the best pitchers," Bell said. "We're going to look for a right-hander that can consistently get a left-hander out, too. That is something we are focusing in on right now, whether that is [Joel] Peralta, [Joakim] Soria, [Todd] Wellemeyer, whoever. We're trying to find who can get a left-hander out, so we don't have to match up every inning late."
Musser gave up a home run to Garret Anderson, an Angels left-handed hitter, in his previous outing.
"We want to see how effective he is against left-handers," Bell said. "Because of the decisions we have to make up here, we're just not going to have the innings for him."
Jorge De La Rosa, who is also left-handed, has done nothing to dispel his chances of being the No. 4 starter -- he gave up two runs on three hits, walked none and struck out three over four innings in the Royals' 7-6 victory over the Giants on Thursday.
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"If he doesn't make our rotation, there is a chance he would be in our bullpen," Bell said. "Right now, he still figures in the rotation. Bannister and Greinke for me are kind of even (for the fifth slot). De La Rosa, again for me, is a little bit ahead because we liked what we saw last year. We didn't see Bannister and Greinke really at all last year. Soria is throwing really well. He is not with Bannister and Greinke, but he is a consideration for the rotation."
Duckworth gave up three solo home runs in three innings against the Giants, but struck out five.
"Honestly, we want to find a spot for Duckworth on our staff somewhere," Bell said. "That is what we would like just because he is a strike thrower. Whether that happens, I don't know. There are a lot of people we like that we're trying to fit on our club. Unfortunately, we don't know how that is going to work out. [Minor League] Options may come into effect as well. This guy has an option, this one doesn't. That is not necessarily the best way you want to do it at times, but to your depth we might have to do it that way."
Bannister and Greinke both have Minor League options left, while De La Rosa does not. Duckworth, who had elbow surgery last August 15, is a non-roster invitee.
"We're not at a point where we feel like making a decision just yet," Bell said of the pitching staff.