Seattle ace Bryan Woo left Friday night's start in Houston early, appearing to show discomfort while throwing warm-ups.
After holding the Astros to one hit over five innings, the right-hander was on the mound warming up when, after a throw, he looked down at his glove and appeared to show frustration as the team's training staff came out.
The 2025 All-Star walked off the mound and into the dugout with a trainer with the Mariners leading 2-0 in the bottom of the sixth.
Woo, 25, entered the game with a 14-7 record and 3.02 ERA. Before Friday, he failed to go at least six innings only twice in 29 starts.
The game is the first of a three-game set between teams tied for first in the American League West at 84-69 with nine games remaining.
Righty Eduardo Bazardo replaced Woo and pitched two scoreless innings as Seattle increased the lead to 4-0 after 7 1/2 innings.
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