The American League playoff picture, and particularly the tight AL West race, may have received a massive variable Friday night.

Mariners ace Bryan Woo left the 4-0 win over the Astros with pec tightness and is "likely" set to undergo an MRI exam on Saturday, according to Seattle manager Dan Wilson.

The Mariners lead the Astros by one game in the division and are tied with the Tigers for the No. 2 seed, which comes with a bye.

"Obviously concerned in some ways but I think in other ways, Bryan's a tough cookie and it was smart that he let us know and there was no reason to push it and I think we'll just find out (Saturday)," Wilson said after his team improved to 85-69. "Too many unknowns at this point."

Woo is one of the AL's best starters, and his five shutout innings Friday improved his record to 15-7 while lowering his ERA to 2.94.

Any potential Seattle run likely needs him at his best, and this injury scare could potentially be a humongous blow should he be forced to miss any time.

Wilson said Woo had notified the dugout he had felt a little something between the fifth and sixth innings, but he still felt fine enough to start the next inning.

He took the mound for warmup pitches but something did not seem right, with the broadcast showing how a trainer walked onto the field right after one throw in which Woo seemed uncomfortable.

Woo allowed just one hit and one walk while striking out seven to best Hunter Brown in a duel.

"I just felt it at the end," Woo said, per ESPN. "I thought it was smart to not try to push it."

Eduardo Bazardo replaced Woo in a 2-0 game and pitched two scoreless innings, and the Mariners now need just one more win in this series to claim the season tiebreaker versus Houston.

He added: "It didn't feel like it was best for the team. I felt like Baz was ready to go. We'll get some stuff done (Saturday), and I'll know more. I really don't know much."

Seattle is aiming to return to the playoffs for just the second time in the last 24 years and has relied on its excellent pitching staff, with Woo potentially starting Game 1.

Any injury that requires an injured list stint could compromise his availability for the playoffs.

Seattle plays these two final games in Houston before finishing the season at home against the dreadful Rockies and the Dodgers.

Bryan Woo to undergo MRI for 'pec tightness' in potential Mariners...