Taillon (2-3, 4.10 ERA) last pitched on May 1. The 27-year-old, who underwent Tommy John surgery in 2014, was on the injured list for several weeks and then experienced a setback while trying to rehab last month.
The surgery has not been scheduled, and there is no timetable on a potential return date.
"We all, including all the experts that [Taillon] did see, felt that we could salvage some of the season -- that he could return this year," Pirates director of sports medicine Todd Tomczak told reporters before Friday's 8-4 win over the New York Mets. "You always want to avoid surgery at every cost if you're a professional athlete because there are no guarantees."
The injury is the latest setback for Taillon, who in addition to missing all of 2014 due to surgery fought testicular cancer in 2017. He bounced back to put together the finest season of his young career in 2018, going 14-10 with a 3.20 ERA.
The Associated Press contributed to this report.