OAKLAND -- Andrés Muñoz has been dealing with a lower back issue for weeks. But after Seattle¡¯s leverage reliever was front-and-center for a home-plate collision on Tuesday night, he could no longer pitch through it.

Seeking his 13th save, Mu?oz was instead forced to leave with two outs in the bottom of the ninth inning of the Mariners¡¯ 4-3 win over the A¡¯s after barreling into Max Schuemann when attempting to prevent a run on a wild pitch of his own doing.

Mu?oz attempted one warmup throw after the run scored, under the supervision of Mariners manager Scott Servais and assistant athletic trainer Kevin Orloski, after which the group determined he was not at 100%.

Mu?oz will undergo an MRI to detail the severity of the issue, and preferred not to speak with reporters until after deciphering the results.

"It's something he's able to work through and be able to pitch, and he's been so effective,¡± Servais said. ¡°He's had an unbelievable season for us. But, an awkward fall on his back. So, again, we'll take a look at it, get some pictures of it, see what it looks like and take it from there."

Aside from his back, Mu?oz awkwardly rolled over his right foot when fielding the underhand relay from catcher Cal Raleigh as Schuemann was sliding feet-first to score, after Mu?oz¡¯s first-pitch slider to JJ Bleday bounced near the lefty¡¯s front foot and reached the backstop.

After the one warmup pitch, which was more of a light toss than a legitimate offering, Mu?oz shook his right hand in disapproval then walked with Servais and Orloski back to the visiting dugout, where he remained momentarily before receding to the clubhouse. It¡¯s also worth noting that Mu?oz was seen with a light smile when conferring with Mariners director of pitching strategy Trent Blank in the dugout.

¡°We're always concerned,¡± Servais said. ¡°He's so good. He's one of the dominant relievers in this league and we lean on him heavily with as many close games as we play, but ultimately we've got to do this thing for the player.¡±

Mu?oz is coming off a month in which he went 7-for-7 in save opportunities, with one earned run in 13 1/3 innings (0.68 ERA) to go with one walk and 19 strikeouts among 47 batters faced. Since his worst outing of the year on April 5 at Milwaukee -- when he walked the bases loaded before walking in the winning run -- he¡¯d been scoreless in 19 of 21 outings.

On Tuesday, Mu?oz was called on to preserve a 4-2 lead. He allowed a one-out single to Schuemann and a two-out knock to Miguel Andujar that advanced Schuemann to third, setting up the fateful play.

After Mu?oz's unexpected departure and with the game on the line, Tayler Saucedo was brought in to lock down the victory, inducing a weakly hit comebacker to end an eight-pitch battle with A¡¯s cleanup man Brent Rooker, after walking Bleday to put the potential winning run on first base.

¡°It's obviously challenging,¡± Saucedo said. ¡°It catches you off guard a little bit, but again, we've been there before. Once you know that your name could potentially be called, you just turn it on.¡±

Regardless of the MRI results, Mu?oz is Seattle¡¯s latest reliever to be banged up this year, having mentioned in passing a few weeks back that his lower back had been bothering him -- but not to the point where he couldn¡¯t pitch.

Entering the season, the Mariners were banking on him anchoring a three-headed group along with Matt Brash and Gregory Santos. But Brash suffered an elbow injury in Spring Training that led to Tommy John surgery last month, and Santos has also been out since spring but is progressing from a right latissimus dorsi strain. The Mariners are optimistic that he¡¯ll be ready by July.

Seattle is also without lefty Gabe Speier, who was placed on the IL on Saturday with a left rotator cuff strain. He¡¯d been their best leverage lefty until a skid over the past few weeks unrelated to the injury. Saucedo is now in that role, after missing three weeks himself with a right knee hyperextension.

The Mariners¡¯ bullpen has been a strength as they sit in first place in the American League West with a 5 1/2-game lead -- their largest lead since Aug. 15, 2003 -- as they advanced to 25-0 on Tuesday when leading after seven innings.

But an issue with their best arm is certainly a cause for concern.