Martinez, who is working his way back from a shoulder injury, threw a 30-pitch side session Monday, but the Cardinals are worried they won't have time to stretch him out for a potential postseason push.
"If you're going to go down that starting path and you're starting at ground zero, the likelihood of him being back would be sometime in mid-September," Cardinals president John Mozeliak told reporters, according to MLB.com. "And we've done that twice this year and ended up in the same place after a few starts."
Cardinals interim manager Mike Shildt did not comment on a specific role out of the bullpen for Martinez, but confirmed that the right-hander is "probably looking like a [bullpen] weapon for us the rest of the year."
Martinez will throw one more side session in the next few days and could return to the active roster soon.
"I feel great right now," Martinez said, according to MLB.com. "I'm comfortable with all my pitches."
Martinez was the Cardinals' Opening Day starter but has been on the DL three times this season. The two-time All-Star, who hasn't pitched out of the bullpen since 2015, is 6-6 with a 3.41 ERA in 18 starts.