Arroyo said he believes the injury occurred during his May 13 start, when he outdueled Stephen Strasburg by allowing one run while tossing a complete game.
Arroyo made six starts after that, using a lot of pain medication and making the best of diminished velocity.
The day after his June 15 start against the Dodgers, he had an MRI taken and it showed the ulnar ligament had been torn completely off the bone.
"I just figured it would be a lot of swelling, probably my flexor mass would be irritated and it was, it had a lot of swelling in there as well," Arroyo said. "But as soon as doc looked at it, he said, 'Man, the entire thing is gone. It's just not there.' Which I was completely shocked about, because like I said, I've had pains like that before and I've always found a way."
Arroyo tried to find one again this time. To try to avoid surgery, he elected to rest the elbow for a few weeks before he started a throwing program.
"I fired it up the last three days and I could throw 120 feet and I could probably go out there and pitch, but it just won't come back fast enough," Arroyo said. "So I'd have to pitch every 10 days and take nine days to get it healthy. It just wasn't going to work."
Over his 15-year big league career, Arroyo had made 405 appearances and 369 starts without interruption.
The 37-year-old signed a two-year contract with the D-backs in February at which time an MRI showed no ligament issues. Arroyo is owed $9.5 million next year and there is a team option at $11 million for 2016, which includes a $4.5 million buyout.
Arroyo's hope is to return by next June and continue to pitch for another couple of seasons after that.
"If this was the last year that I was going to play, I would just go gut it out and pitch and it would hurt and I'd try to find a way to win at 82 miles per hour, but I don't think it is," Arroyo said. "I think my body's a lot younger than my age is. I think I'm probably closer to 32-33 as far as the way I feel in comparison to most guys my age. So I don't think it's going to be a problem coming back."
That is if he can survive being on the disabled list.
"I'm so irritated," he said. "You have no idea. I spent two decades trying to avoid the disabled list, and now I've got to sit on this thing for a year. It's going to be a long year."