Astros closer Josh Hader underwent testing on Monday after experiencing shoulder discomfort during a workout that left him available to pitch in Houston's 7-6 win against the Boston Red Sox (box score). Manager Joe Espada instead navigated the ninth inning using a combination of relievers: right-hander Enyel De Los Santos and lefty Bennett Sousa, who was credited with his fifth career save (and fourth of the season) after striking out the only batter he faced.

"We're waiting on those results, and we should have something more tomorrow," Espada told reporters of Hader's availability moving forward. 

Hader, 31, has pitched just three times in August, with his most recent appearance (on Aug. 8) doubling as his seventh this season in which he's logged four or more outs. He's been highly effective overall, tallying a 2.05 ERA (202 ERA+) and a 4.75 strikeout-to-walk ratio.

It's too soon to know if Hader will require a stint on the injured list, but an absence of any length would be an unwelcome development for an Astros team accustomed to this song and dance. The Astros are currently without a number of key contributors, including third baseman Isaac Paredes, outfielders Yordan Alvarez and Jake Meyers, and starters Ronel Blanco, Lance McCullers Jr., Luis Garcia, and Hayden Wesneski.

All those concurrent injuries go a long way in explaining why the Astros have seen their lead in the American League West slip away. They'll enter play on Tuesday up by a single game over the Seattle Mariners -- that after maintaining a five-game advantage as recently as July 30. Houston is 5-5 since; the Mariners, conversely, are 9-1 over that stretch.

If Hader is sidelined, expect Espada to continue to employ a committee approach that includes, among other pitchers, Bryan Abreu and Bryan King.