"We've liked him as an organization the last few years having watched him pitch," Phillies manager Pete Mackanin said. "He's had quite a bit of success. I'm glad we get a chance to get a look at him."
The Phillies will be responsible for the remainder of his $1.64 million salary for this season, which comes out to about $270,000 for the final month of the season. Siegrist is arbitration-eligible for two more years, and he can't become a free agent until 2020.
Despite a career 3.03 ERA in five seasons with the Cardinals, Siegrist has struggled this season. He owns a 4.98 ERA and a career-high 1.602 WHIP in 39 appearances.
Some of Siegrist's woes are likely due to injuries. Left forearm tendinitis landed the 28-year-old on the 10-day DL on Aug. 6, and a cervical spine sprain in June cost him a good chunk of time as well.
Any lingering injury concerns didn't scare the Phillies, who were more than happy to give the once dominant southpaw a chance. From 2015-16 with St. Louis, Siegrist posted a 2.44 ERA and a 10.3 K/9 in 136 1/3 innings.
"I don't know much about his injuries, but I know he's available and we'll get a look at him," said Mackanin, noting that Siegrist would be available for Saturday night's game. "I'm anxious to see Siegrist and see where he fits in."
If healthy, Siegrist will help a Philadelphia relief corps that owns a 4.51 ERA this season, which ranks 22nd in the Majors.
Left-handed relief has actually been a strong suit for the Phillies. Siegrist joins Hoby Milner and Adam Morgan, who have combined to give up one run in 25 innings since Aug. 1.
"Those two guys have been outstanding for a month now," Mackanin said. "It's really two nice pieces to have in the bullpen. If we can add another one that's even better."
In a corresponding move, Jerad Eickhoff was transferred to the 60-day DL.
Patrick Pinak is a reporter for MLB.com based in Miami and covered the Phillies on Saturday.