Mariners right-hander Nate Robertson will undergo elbow surgery on Wednesday, aidig the case of the team's top pitching prospect, Michael Pineda (right), as the team's fifth starter.
Robertson's surgery will remove "loose bodies" from his left elbow and won't be able to throw for four weeks after the surgery.
Robertson made two spring starts and was 0-1 with a 7.71 ERA. He'd allowed 11 hits and six earned runs in seven innings pitched, along with seven strikeouts and two walks.
Robertson told the Seattle Times he would likely be ready to pitch for the Mariners by early June. He also said he was told by the team they'd like to keep him going forward. He signed a minor-league deal with the team in January. He was 6-8 with a 5.95 ERA with the Marlins and Phillies last season.
Pineda, a 22-year old right-hander, is the team's top pitching prospect. He's appeared in three games this spring, starting two, and allowed two earned runs in seven innings. He's struck out five and walked three, while allowing five hits.
Pineda can hit triple digits with his fastball and has shown improvement in his slider and changeup. He's expected to pitch again on Thursday against the Royals.
Pineda is already on the team's 40-man roster.
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