"It's like pitching with a rock in the bottom of your neck and the upper part of your shoulder," A's manager Bob Melvin said.
Gray was subsequently having trouble finding his normal arm slot and getting extension on the ball, translating to command issues.
He first experienced tightness in the area ahead of his May 15 start in Tampa, but he made it through 5 2/3 innings that day -- allowing six runs (three earned) -- with a slight nagging pain that dissipated during his ensuing bullpen session. But it locked back up on him during his first warmup pitch in the fourth inning of Friday's start against the Yankees, and he managed to record just one out in the frame before departing.
Gray, 26, was injected with a shot -- half cortisone, half anti-inflammatory -- before the game even ended, and he said Sunday morning he already feels better, leading him to believe he won't need to miss more than the minimum 15 days.
"I don't think it's anything too serious, but we just made the decision to go ahead and knock this thing out and not let it linger," Gray said. "We'll take these next few days and really work on the back of my shoulder and let the area calm down again, and hopefully when these 15 days are up, never look back from there."
"It's something we need to get out of there and iron it out so he can throw the baseball where he wants to," Melvin said. "We're going to make sure that he gets enough treatment to where when he starts to play catch it feels good. He'll throw a couple of bullpens, maybe a simulated thing, and then maybe a rehab start, so whether it's 15 days or shortly thereafter, we'll see."
The A's have a Major League-leading 13 players on the disabled list, five of them starters: Gray, Henderson Alvarez, Chris Bassitt, Felix Doubront and Jarrod Parker. Gray's replacement, in line to pitch Wednesday in Seattle, is likely to be right-hander Zach Neal, though Melvin wasn't ready to make any announcement Sunday, instead calling on left-hander Daniel Coulombe from Triple-A Nashville for relief help in the meantime.
Neal, who made his big league debut with the A's in a relief stint in Boston on May 11, is 5-1 with a 2.53 ERA in seven starts for Nashville.
"You know you're going to deal with injuries over the course of the season," Melvin said. "Middle of May, having 13 is probably pretty severe, but not much we can do about it. Just give someone else an opportunity."
This is the first career DL stint for Gray, who hasn't looked like himself nearly all season, unable to get through four innings in three of his last five starts. During that stretch, he has a 10.38 ERA and a .337 opponents' batting average.
His 6.19 ERA on the season is third-highest in the American League, and nowhere near the 2.88 mark he posted in his first three seasons with the A's across 76 games, 74 of them starts.
The 2015 All-Star "wants to be out there, he wants to pitch," Melvin said. "The last thing he wants to do is not only not be there every fifth day for us but on his starts have to come out of games early because he's throwing a lot of pitches and not throwing the ball where he wants, so we have to get him ironed out."
"I think the line gets drawn when you're not really being able to contribute and help the team win," Gray said. "It's better to go ahead and try to miss a couple starts and knock this thing out. In the long run, it'll benefit everyone. When you're hurting more than you're helping, it's something where you gotta take a step back and look at and get yourself right to help the team."