Mariners left-hander Drew Smyly has been diagnosed with a flexor strain in his left arm and will miss the first six to eight weeks of the regular season, tweets Bob Dutton of the Tacoma News Tribune. In his place, left-hander Ariel Miranda will step into the Seattle rotation and start the team’s fourth game of the year, Dutton adds.

Ryan Divish of the Seattle Times notes that Seattle doesn’t think surgery will ultimately be necessary for Smyly, but the left-hander will head out for second and third opinions on the injury from different doctors in order to be sure (Twitter links).

The loss is, obviously, a dreadful blow for the Mariners to suffer so close to Opening Day. Smyly was utterly dominant when pitching for Team USA in the World Baseball Classic and was expected to play a significant role in the Mariners’ rotation in 2017.

While he’s coming off a dismal 4.88 ERA in 175 1/3 innings with the Rays last season, Smyly averaged 8.6 strikeouts and just 2.5 walks per nine innings pitched in that time as well. He’s demonstrated improved strikeout and walk rates with Tampa Bay over the past two seasons and, as an extreme fly-ball pitcher moving to the spacious Safeco Field with what looks to be a terrific outfield defense, was viewed by many (myself included) as a potential breakout candidate this year.

With Smyly on the shelf, the Mariners’ rotation will consist of Felix Hernandez, Hisashi Iwakuma, James Paxton, Miranda and offseason acquisition Yovani Gallardo (who, like Smyly, is hoping for a rebound campaign in his new environs). Seattle picked up Miranda in the trade that sent Wade Miley to the Orioles last season, and Miranda enjoyed success down the stretch with the M’s (3.88 ERA in 58 innings — albeit with less-encourgaing peripheral stats). As such, they won’t be plugging a complete unknown into the starting five, but Miranda is nonetheless a step down in terms of what can be reasonably expected out of that rotation slot.

Miranda is hardly the only depth option the Mariners have, however. On the contrary, GM Jerry Dipoto made a point of adding a number of starters with big league experience to the 40-man roster this winter, picking up Dillon Overton, Rob Whalen and Chris Heston on the trade market. Should Miranda falter, or should another starter fall to an injury, any of that bunch could conceivably step into the Seattle rotation as well.