BOSTON (CBS) – Will Middlebrooks missed Monday night’s series opener in Oakland with a sore hamstring, and could end up missing a lot more action with the injury.

Red Sox manager Bobby Valentine told reporters the rookie third baseman was showing signs of improvement, but it was too early to rule out a potential trip to the disabled list.

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“The latest on Will is that he feels better today than he did yesterday and better than he expected to be today. That’s the only news we have. That’s good,” Valentine said Monday evening. “It’s not discolored, and that’s good. But it’s going to be some days.”

View: Red Sox Injuries

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“We won’t let the calendar dictate [when he comes back],” said Valentine. “We’ll just see how he feels. Maybe DL, maybe a few days, it’s just one of those injuries. I think we’ve had them before.”

With 13 pitchers on the roster, Valentine hinted at a move to bring up another infielders. Nick Punto started in Middlebrooks’ place Monday night, going 0-for-3 in the 6-1 loss to the A’s.

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In 48 games this season, Middlebrooks is batting .298 with 10 home runs and 37 RBI.