ST. PETERSBURG -- Four games into the season, the Rays’ starting pitching depth is already being tested.

Last August, Tampa Bay ace Tyler Glasnow underwent Tommy John surgery. In Spring Training, the Rays lost top prospect Shane Baz (arthroscopic right elbow surgery) for at least the first month of the season. On Monday afternoon, they placed left-hander Ryan Yarbrough (left groin tightness) on the 10-day injured list, although he’s expected to return soon. That will result in pitching prospect Tommy Romero making his Major League debut on Tuesday night.

Then came another blow Monday night, when right-hander Luis Patiño exited the Rays’ 13-2 loss to the A’s at Tropicana Field in the first inning due to a left oblique strain.

“We've had two starters go down pretty quick lately, but we'll regroup,” Rays manager Kevin Cash said after their first loss of the season. “We've got enough pitching that should be able to cover us. Luis, we've got to get him healthy because he's a big part of what we're trying to do here, and his stuff, when he's right, is really, really impressive.”

Making his season debut, Patiño threw only 13 pitches before he had to leave the game. The 22-year-old righty dealt with a sore throwing shoulder in Spring Training, which set his schedule back a bit, but said he felt good physically heading into his first start.

“I felt fine when I was in the bullpen, when I was throwing out there,” he said through interpreter Manny Navarro. “The first few batters, I felt fine. I felt really good.”

Patiño seemed uncomfortable after his 12th pitch Monday night, a slider that landed in the zone for a called strike. He crouched behind the mound for a moment, seemingly gathering himself for the next pitch, then stepped down the mound in even more obvious discomfort after throwing a 92.9 mph fastball that Oakland catcher Sean Murphy took for a strike.

Patiño squatted in front of the pitching rubber, which drew Cash and assistant athletic trainer Aaron Scott from the dugout. After a brief conference on the mound, Patiño left the game.

“I felt something definitely on that [second-to-last] pitch, and I thought I could maybe work through it, stretch it out, maybe it’s just a little bit tight,” he said. “But once I let the next pitch go, that’s when I really felt it. And that’s when I had to stop, because it wasn’t feeling too good anymore at that point.”

Patiño will undergo further testing and an MRI before the club makes any plans or sets a timeline for his return. When considering previous pitchers who have dealt with strained obliques, though, he could be sidelined around two months depending on the severity of his injury and the amount of time it takes for him to rebuild his arm strength.

“We’ll see what the MRI says,” Patiño said. “For now, I’ll try to take all the negative stuff out of my head and try to think a little more positive and see what happens tomorrow.”

Things didn’t get much better for the Rays after Patiño’s early exit.

Thrust into an unenviable position, right-hander Chris Mazza went straight from the bullpen to the mound and plunked Murphy with his first pitch. Mazza then served up back-to-back home runs to Seth Brown and Chad Pinder before getting out of the first inning. After two singles and an error by second baseman Taylor Walls in the second inning, Mazza gave up a three-run homer to Elvis Andrus that made it an 8-0 game.

Meanwhile, the Rays couldn’t get anything going against A’s starter Paul Blackburn, who struck out a career-high seven over five scoreless innings. Wander Franco, batting .600 through four games, scored both of Tampa Bay’s runs while recording his third three-hit game of the season. Franco singled and scored in the sixth inning on a fielder’s-choice grounder by Josh Lowe, then doubled in the eighth and scored on Ji-Man Choi’s single to right.

“Nothing really went our way,” Cash said. “We were not as crisp as we've kind of come to be, so just a game you'd like to move past pretty quick.”

Mazza wound up needing 73 pitches to cover three innings, putting the Rays’ bullpen in a tough spot amid a season-opening stretch of playing on 13 straight days. Right-hander J.P. Feyereisen covered 1 1/3 innings, Ryan Thompson worked a scoreless inning in his first Major League outing since June 27, Jason Adam pitched one frame and outfielder Brett Phillips handled the final two innings as only he could, providing some entertainment on a night Tampa Bay would otherwise prefer to forget.

“Over a course of 162 games, games like this are going to happen,” Phillips said. “It's a marathon, and we'll come back tomorrow with high spirits. We know what we can do, so just looking forward to getting back here tomorrow and doing our thing.”