HOUSTON — What was shaping up to be maybe the best day of the season for the Mariners turned, suddenly and ominously, into its most dreaded.

Bryan Woo, the Mariners’ 25-year-old All-Star right-hander, exited Friday night’s game against the rival Houston Astros with an unspecified injury after throwing several warm-up pitches before the bottom of the sixth inning.

Woo motioned to the dugout after a warm-up pitch and soon left the mound with team trainer Kyle Torgerson.

The team had no immediate update on Woo’s condition.

The injury to their emerging ace turned what should have been an exuberant 4-0 victory over the Astros into an evening of anxiety and uncertainty.

Julio Rodríguez’s first-inning home run off Houston ace Hunter Brown put the Mariners in control early, and three more solo homers — from Eugenio Suárez, Victor Robles and Josh Naylor — put the Mariners (85-69) alone atop the AL West, one game ahead of the Astros (84-70) with two games remaining here in Houston.

The Mariners have won 12 of 13, and they moved into a tie with the Detroit Tigers for the AL’s No. 2 seed, which grants a first-round bye.

The M’s are playing their best baseball of season with October just around the corner. There’s a growing sense — both in their clubhouse and from those around the sport — that this Mariners roster could be better positioned than any other AL team to make a serious run at a World Series berth.

Woo, the Mariners’ best and most durable arm this season, had thrown five shutout innings in one of his most effective outings of the season.

He retired the first 10 batters he faced, touching 97.9 mph early, and allowed only one hit with one walk.

Woo’s injury, if significant, would seriously alter the complexion of the Mariners’ postseason plans.

Woo has emerged this season as the Mariners’ workhorse, a first-time All-Star who leads the team in virtually every pitching category, and notably in innings pitched.

That’s an achievement for a young pitcher who has battled through several injuries early in his career — he had Tommy John surgery while still in college in 2021, and several stints on the injured list during his first two MLB seasons — and for a Seattle rotation that treaded water for much of the season while Logan Gilbert, George Kirby and Bryce Miller were on the injured list.

If Woo is unable to make his next start, Emerson Hancock could be an option. He moved to the bullpen earlier this month, but could conceivably give the Mariners some length.

Rookie Logan Evans, who made 15 starts for the Mariners this season, remains on the injured list with shoulder inflammation.

Eduard Bazardo took over for Woo in the sixth inning Friday and pitched perfect innings, continuing his breakthrough season out of the bullpen.

Matt Brash worked a scoreless eighth inning and Andrés Muñoz worked the ninth in a non-save situation to complete the shutout.

Mariners pitchers have posted 11 shutouts this season. They also shut out the Kansas City Royals on Thursday, the second time this season they’ve posted consecutive shutouts (they shut out the Pirates in three straight games in early July).

Rodríguez launched a homer to the back of the Crawford Boxes in left field, turning on a 96.5-mph sinker from Brown that was several inches off the plate inside. That quieted the sold-out crowd of 41,471 at Daikin Park early on, and those fans had little to cheer for the rest of the night.

Suárez hit his 47th homer in the fourth inning on a low curveball from Brown.

Robles hit his first homer in the season off the high wall in left field in the seventh, and Naylor added the fourth solo blast of the night in the eighth.

It was Naylor’s 20 homer of the season, making him the rare first baseman to post a 20-homer/20-steal season.

Coming into this series, the Mariners and Astros had split their first 10 games of the season. That means the team that wins this three-game series will hold the edge in any potential tiebreaker scenarios at the end of the regular season.

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