OAKLAND -- The exuberance that oozed through each A’s player leaving the clubhouse Thursday night following one of the team's most exciting wins of the season was replaced with a feeling of somberness when they returned Friday afternoon.

The Office of the Commissioner of Baseball announced Friday that A’s pitcher Frankie Montas has been dealt an 80-game suspension without pay for testing positive for Ostarin, a performance-enhancing substance in violation of Major League Baseball’s Joint Drug Prevention and Treatment Program. The suspension is effective immediately.

“I am deeply saddened to confirm that MLB recently notified me I have tested positive for Ostarine, a prohibited substance under MLB’s Joint Drug Agreement,” Montas said in a statement, via the MLB Players Association.

Montas said he unknowingly ingested the banned substance as part of an over-the-counter supplement he purchased within the United States.

“While I never intended to take any prohibited substance, I unfortunately and unknowingly ingested a contaminated supplement that I had purchased over the counter at a nutrition store here in the United States,” the statement continued. “That said, I respect MLB rules and understand my responsibilities under the Joint Drug Agreement, and accept full responsibility.

“I sincerely apologize to the A’s organization, the fans, my teammates, and my family for this mistake. My hope is to be able to return to the A’s alter this season and contribute as best as I can.”

The suspension comes less than 24 hours after Montas put together arguably the most impressive outing of his career, holding the Rays to one run over eight innings with nine strikeouts and no walks in a 5-4 walk-off victory.

Montas, 26, was in the midst of a breakout campaign after bouncing around organizations to begin his young career. The righty owns a 9-2 record and a 2.70 ERA across 15 starts in 2019, and, according to FanGraphs, his 2.9 WAR is fourth best among qualified starters.

“I think it was a shock to everybody today,” A’s manager Bob Melvin said. “We met with Frankie as a team. He spoke and was very remorseful and is going to accept what he has to deal with. He feels bad about it.”

A’s general manager David Forst learned of the news this morning and met with the right-hander shortly after. He said Montas had known about the positive test “for a while now” and went through an appeal process, which was unsuccessful.

“It’s disappointing on a number of levels,” Forst said. “Personally disappointed in Frankie and disappointed for his teammates who have worked very hard over the first half of the season and are playing very well of late.

“He was apologetic and feels like he let his teammates and organization down. I appreciated that sentiment, but at that point, I don’t think there was much we could do.”

Oakland will now have to trudge on without Montas as it seeks a second consecutive postseason berth. Montas will be able to return, at the earliest, by the sixth-to-last game of the regular season and would be disqualified from the playoffs should the A’s make it that far.

What’s next?
The A’s recalled right-hander J.B. Wendelken from Triple-A Las Vegas to take Montas’ place on the 25-man roster, but it’s still unclear who will fill the now-vacant spot in the starting rotation. Aaron Brooks, now in the bullpen after beginning the year in the rotation, could move back and fill Montas’ role. Other options at Triple-A include Daniel Mengden and Paul Blackburn, both right-handers who have started games for Oakland this season.

The A’s No. 1 prospect, Jesus Luzardo, could soon become another option for the rotation. The left-hander is set to make his first Triple-A start of the season Saturday as the club looks to build his pitch count up to 100 in his return from a left shoulder strain.

“We’ll figure it out over the next few days,” Forst said. “If there’s something this group has shown over the last year and a half, it’s that ‘next man up’ is their motto. Whether it’s because of performance, injuries or something like this, every guy in the organization has shown the ability to step up. It led to a lot of great things last year, and I think they’re going to feel that same responsibility this time around.”

Manaea close to rehab assignment
Sean Manaea (shoulder) threw 60 pitches in a simulated game at the A’s Minor League complex in Arizona on Thursday and came away feeling healthy.

“Everything felt good,” Manaea said. “Still able to throw my changeup for strikes, and the slider is coming along.”

Manaea will meet with A’s head athletic trainer Nick Paparesta in the coming days to determine a start date for a Minor League rehab assignment, which he expects to begin in the next week. After targeting the MLB All-Star break for a potential return to the big leagues, Manaea said he should be right on track to meet that goal.

“I can’t wait to get out there and face some actual competition in a game atmosphere,” Manaea said. “Really excited about that.”