The Houston Astros will be without one of their key bullpen arms for the stretch run.

Right-hander Ryan Pressly will be sidelined for the next four-to-six weeks after he undergoes arthroscopic knee surgery on Friday, general manager Jeff Luhnow told Chandler Rome of the Houston Chronicle.

Pressly was placed on the injured list and right-handed reliever Joe Biagini was recalled in a corresponding move.

The 30-year-old was placed on the 10-day IL on July 27 with right knee soreness but was activated earlier this month. After experiencing more soreness during a recent outing, the Astros imaged the knee and found a "small issue" that was not present during earlier MRIs, Rome adds.

Luhnow said there's still a chance that Pressly appears in a regular-season game prior to the playoffs.

Pressly was acquired by the Astros from the Minnesota Twins last year for a pair of minor-leaguers. The reliever proceeded to become a crucial piece of Houston's relief corps, as he authored a 0.77 ERA and 1.49 FIP over his final 23 1/3 innings of the season.

As the go-to reliever behind closer Roberto Osuna, Pressly has posted a 2.50 ERA and 2.83 FIP over 50 1/3 innings this campaign.