Webb, 26, had been one of the most effective pitchers in Atlanta's bullpen, with a 4-0 record, 2 saves and a 1.39 ERA in 36 appearances. But he had not pitched in the big leagues since going on the 10-day IL with an elbow impingement July 13.
"His elbow still isn't right," Braves manager Brian Snitker said before Tuesday's game against the Miami Marlins. "They don't know [what it is]. They'll have him looked at again, do all the tests."
Webb began a rehab assignment at Triple-A Gwinnett on July 31, but he struggled in 10 appearances, posting a 6.97 ERA.
"You could tell something wasn't right just by the results he was getting in Triple-A," Snitker said. "They'll look at him further, re-evaluate him, and if something needs be done, then get it done now."
The Braves aren't sure if Webb will need surgery, which might delay his return in 2020.
"Hopefully by shutting him down this year, he'll be ready to go for spring training," Snitker said.
Webb was recalled from Gwinnett and shifted to the 60-day IL to make room on the 40-man roster for outfielder Billy Hamilton, who was claimed off waivers Monday from Kansas City.