(Photo credit: Stephen Brashear-Imagn Images)

Athletics rookie pitching standout Jacob Lopez was diagnosed with left forearm tightness after his uncharacteristic rough outing on Sunday, manager Mark Kotsay said following the 11-4 loss to the host Seattle Mariners.

Lopez fell to 7-7 with a 4.08 ERA after allowing nine runs on six hits -- including a pair of historic two-run homers by Mariners catcher Cal Raleigh -- and a career-high six walks with two strikeouts in two-plus innings.

The left-hander was 4-0 with a 0.70 ERA in four previous starts this month. Lopez did not allow a run in four starts from July 26 to Aug. 12 before yielding three runs (two earned) on four hits and one walk while fanning eight in a 6-3 win at Minnesota on Tuesday.

'I felt it a little bit after the start in Minnesota,' Lopez said of the forearm discomfort, per mlb.com. 'I did some more things in between starts and thought I was good.'

The walks, and Raleigh setting the major league record for most home runs in a season by a primary catcher with his 48th and then 49ths blasts, were all Kotsay needed to see. The pitcher was pulled after facing four batters (two walks, two singles) in the third inning.

'The lack of command showed back up,' Kotsay said, per mlb.com. 'When we took Jacob out, he didn't really feel great ... It might have led to the outing. But sometimes you're on a roll and you have a bad one.'

Lopez, 27, didn't blame his ailing arm, only 'poor pitches.'

'Just one of those days where I was the worst player on the field,' said Lopez, who has 113 strikeouts and 37 walks in 92 2/3 innings over 21 games (17 starts).

The Athletics acquired Lopez and left-hander Jeffrey Springs from the Tampa Bay Rays last December for two minor leaguers and a draft pick. Lopez missed the 2022 season after having Tommy John surgery in 2021 while a Rays prospect.

'I have no idea right now,' Lopez said of the extent of his arm injury. 'I've already had a TJ. This feels different. It feels more in my flexor, so, we'll see how it goes.'

--Field Level Media