HOUSTON — In a substantial blow to their well-performing bullpen, the Houston Astros placed southpaw Steven Okert on the 15-day injured list Friday with a right hamstring strain.The IL stint is retroactive to Sept. 1, meaning the earliest Okert could return would be Sept. 16 against the Kansas City Royals — the Astros’ 152nd game of the season. On Friday, Okert described his strain as “pretty minor” and sounded optimistic he could return after a minimum stint.“Obviously it sucks,” said Okert, a 35-year-old journeyman having the best season of his professional career. “I was having a good year, doing my best to help us win games out there. Pretty let down about it, but just got to get to work on the rehab side and try to get back as soon as possible.”Okert has morphed into an invaluable piece of Houston’s bullpen both on and off the field. His easygoing persona has made him one of the leaders inside the Astros’ clubhouse while his performance in leverage innings is one of the few constants in Houston’s otherwise inconsistent season.Who the Astros should hire as their next GMKeith LawOkert owns a 2.13 ERA and 0.87 WHIP across 68 appearances. No Astros reliever has made more, and only three in all of MLB have logged more outings than Okert. When Okert didn’t appear in close games against the Chicago White Sox on Wednesday and Thursday, it seemed team officials were trying to give him a much-needed break.Instead, Okert said he suffered the hamstring strain while doing some warm-up exercises during Wednesday’s game. Imaging on Thursday revealed the strain, which forced Houston to place him on the injured list.“I couldn’t believe it,” Okert said. “Out of all the things, getting ready to go pitch was the thing that got me.”Okert has filled almost every non-closer role in the Astros’ bullpen — from set-up man to fireman to opener. Most recently, Okert has settled into a seventh-inning role behind set-up man Bryan Abreu and closer Josh Hader.According to both Baseball-Reference and FanGraphs, Okert has been Houston’s second-most valuable reliever this season, trailing only Hader, who just earned American League Reliever of the Month honors in August.Okert has allowed just 10 of the 36 runners he’s inherited to score. His neutral platoon splits permitted manager Joe Espada to unleash Okert against any pocket of an opposing lineup. Okert collected more than three outs in 12 appearances and pitched on back-to-back days 14 times, demonstrating how durable he’s become.“Obviously we’re going to miss him,” Espada said. “He’s been a big part of our bullpen and gotten some big outs for us, but we have guys that are going to step up and pick him up.”Abreu and Hader’s presence in the final two innings will soften the blow of Okert’s injury, but the bridge to both of them is suddenly unstable. Rookie standout AJ Blubaugh should see more leverage innings in Okert’s absence, along with southpaw Bennett Sousa.Sousa, however, is surrendering a 1.019 OPS in 51 plate appearances against left-handed hitters. Blubaugh, who is right-handed, has limited lefties to a .573 OPS, which could put him ahead of Sousa in the late-game hierarchy.