Gonzalez, 22, is the No. 15 prospect in the organization, according to MLBPipeline.com. He is 1-1 with a 3.57 ERA in three starts this season with Triple-A Lehigh Valley. He's allowed 20 hits and two walks and struck out nine in 17 2/3 innings.
"I love Severino," Phillies assistant general manager Benny Looper said. "He's got a real good feel for the baseball -- where it's going with all of his pitches. He can't get by with mistakes -- he's not a power guy -- but he can pitch. We simply felt like he was the best option."
None of the Phillies' highly-regarded pitchers in Double-A Reading received consideration for the start.
"They need experience," Looper said about the Double-A rotation, which includes Aaron Nola, Jesse Biddle, Zach Eflin, Tom Windle and Ben Lively. "They need success. And I like for guys to get Triple-A experience. There has been success calling guys up from Double-A, and we probably will again. But the better progression is have some success there, then go to Triple-A and pitch against the more veteran hitters.
"They have some developing to do. We just felt like Severino is the right guy for the spot. It's a tough spot to break in, in St. Louis, but he'll be up for the challenge."
The Phillies need a starter for Tuesday because Sean O'Sullivan is on the disabled list with tendinitis in his left knee. Dustin McGowan started in his place last week, but he is not a long-term solution in the rotation.
Gonzalez could make at least one more start before right-hander Chad Billingsley potentially joins the rotation. Billingsley made a rehab start Saturday with Lehigh Valley.
Todd Zolecki is a reporter for MLB.com. Read his Phillies blog The Zo Zone. Follow him on Twitter. This story was not subject to the approval of Major League Baseball or its clubs.