Oakland Athletics starting pitcher Frankie Montas will miss 80 games due to testing positive for Ostarin, a performance-enhancing substance. (Ben Margot/AP)

Oakland Athletics starter Frankie Montas was suspended 80 games without pay by Major League Baseball after testing positive for a performance-enhancing drug. He can return in late September but is ineligible to pitch in the postseason.

The suspension comes on the heels of Thursday’s dramatic 5-4 win in which Matt Chapman hit a two-out, three-run home run with two outs in the bottom of the ninth inning against the Tampa Bay Rays to clinch Oakland’s fourth straight win. Montas started the game, allowing just one run on four hits in eight innings with nine strikeouts.

“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 through the MLB Players Association. “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. 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 later this season and contribute as best I can.”

“The A’s were disappointed upon learning of this suspension,” the Athletics said in a statement. “We fully support MLB’s Joint Drug Prevention and Treatment Program and we will welcome Frankie back after the discipline has been served.”

Montas is in the midst of a breakout season. In 15 starts, Montas boasts a 9-2 record with a 2.70 ERA and 97 strikeouts over 90 innings.

"It’s disappointing on a number of levels,” Oakland General Manager David Forst said before Friday’s game against the Rays. “Obviously, I’m personally disappointed in Frankie. I’m disappointed for his teammates and for this team who have worked very hard in the first half of the season, played very well of late, and I know have high expectations for themselves going forward. It’s disappointing.”

“He was very disappointed in himself,” Forst added. “He was apologetic. He feels like he let his teammates and the organization down. I appreciated that sentiment, but at that point, there wasn’t much we could do.”

Montas has played a large role in the Athletics’ strong start to the season . Oakland, which hasn’t won a postseason game since 2013, entered Friday just one game behind the Boston Red Sox for the second American League wild-card.

"I think it was a shock to everybody today,” Athletics 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.”

The Athletics recalled pitcher J.B. Wendelken from Class AAA Las Vegas to take Montas’s spot on the 25-man roster, though the team still hasn’t decided how they will fill the suspended starter’s role going forward.

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