"Michael Kopech has informed us of his decision to not participate in the 2020 season," GM Rick Hahn said. "We recognize that reaching this decision is incredibly difficult for any competitive athlete, and our organization is understanding and supportive."
The fireballing right-hander is coming back from Tommy John surgery in September 2018. He hit triple digits on the radar gun during his first spring training appearance.
But Kopech, 24, missed the start of summer camp earlier this month due to what Hahn said at the time was a personal matter.
The Associated Press contributed to this report.