Pryor went on the disabled list April 15 with a torn right lat muscle. The triceps injury occurred during the right-hander's rehabilitation stint with Triple-A Tacoma.
General manager Jack Zduriencik said Sunday that Pryor "worked really hard, but ... he won't pitch the rest of the year."
Pryor began his rehab stint July 16. He recently felt pain while pitching and had an MRI performed.
Zduriencik says, "I'm not sure exactly how he hurt it. He said he felt something back there."
Zduriencik says the doctors "don't like what they saw."
Pryor did not allow a run in seven games this season. He was 3-1 in 26 appearances as a rookie last year.