A's lefty Sean Manaea left Monday's start against Houston in the fourth inning with a strained back muscle.
The team announced Manaea was diagnosed with a strained left rhomboid. The rhomboid muscles are located between the spine and shoulder blade.
It wasn't apparent when Manaea suffered the injury. He threw out Yulieski Gurriel on a comebacker to lead off the bottom of the fourth. Then he threw a fastball for a called strike on Marwin Gonzalez before being visited by head athletic trainer Nick Paparesta and departing the game.
Manaea missed two weeks in mid-June with a strained forearm. He entered Monday ranked second among American League rookies in strikeouts (100) and innings pitched (117 1/3).
The Astros held a 5-0 lead in the seventh.
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