The right-hander is on the 15-day disabled list with elbow inflammation and had been scheduled to throw Friday. He told the AP he had an injection to his elbow a day earlier.
General manager John Mozeliak texted the AP that the timetable had not yet been changed. Mozeliak added he expected to have a better grasp of the situation on Sunday or Monday.
Rookie John Gast is set to make his second start in place of the 35-year-old Westbrook on Sunday against the Brewers. The Cardinals had estimated Westbrook would miss only two starts.
Mozeliak said the injury is "not more serious" than the Cardinals had thought and that Westbrook was "just receiving additional relief." Earlier in the month, the GM had scoffed off the notion of reconstructive surgery.
Westbrook is 2-1 with a 1.62 ERA in seven starts. His DL move is retroactive to May 9.
After being put on the DL, Westbrook said he'd been dealing with elbow pain since spring training that worsened to the point his mechanics had been altered.
Westbrook, who is in the final year of a two-year contract, was 13-11 with a 3.97 ERA in 2012 and missed the postseason with an oblique injury. He won 12 games in 2011, his first full season with St. Louis.
The 24-year-old Gast won his major league debut against the Mets, opening with five scoreless innings before allowing four runs in his sixth and final inning.