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To replace Marshall on the roster, the Reds have recalled right-hander Logan Ondrusek from Double-A.
Marshall made his season debut for the Reds on Sunday, logging a scoreless inning of work. However, he underwent an MRI following the outing and left manager Dusty Baker to believe he wasn't right.
The MRI didn't reveal structural damage in Marshall's arm, but Cincinnati believes he requires some time off. He'll likely be shut down while his tendinitis is treated, and it's unclear what type of time frame the team is looking at right now.
Marshall appeared in 73 games for the Reds last season after he was acquired from the Chicago Cubs. The 30-year-old maintained an ERA of 2.51 with a strikeout-to-walk ratio of 4.63 over 61 innings.
Ondrusek appeared in 63 games for the Reds in 2012, posting an ERA of 3.46. In three seasons in the majors, the 28-year-old owns a career ERA of 3.45.