The Cardinals are now determining the "next course of action" for Hicks. A torn UCL often leads to Tommy John surgery.
Hicks left Saturday's game against the Los Angeles Angels after recording a strikeout in the ninth inning.
The injury was initially diagnosed as triceps tendinitis, and the initial word from the Cardinals sounded promising.
Shortly after Saturday's game, manager Mike Shildt called the diagnosis "favorable," while Hicks said he expected to miss a couple of days or "maybe a week," according to Derrick Goold of the St. Louis Post-Dispatch.
However, that optimism quickly faded once an MRI revealed the torn UCL. The diagnosis puts Hicks' 2019 season in serious jeopardy, and surgery could push a return to late 2020.
The 22-year-old closer is regarded as one of the hardest throwers in baseball, with a sinker that regularly tops 103 mph, according to Statcast. However, his final pitch on Saturday was an 82-mph slider.
Hicks owns a 3.14 ERA and 0.94 WHIP with 31 strikeouts, 11 walks, and 14 saves over 29 appearances this season.