Oakland Athletics starter Frankie Montas has been suspended for 80 games after testing positive for the banned performance-enhancing substance Ostarine, MLB said Friday.

Montas, 26, was in the midst of a breakout season in which he was 9-2 with a 2.70 ERA and 97 strikeouts against 21 walks in 90 innings. He will not be eligible to return to the A's until the end of September and will not be eligible to pitch in the postseason.

After Montas tested positive, he argued that he took a tainted substance not intended to enhance his baseball performance. While the argument was credible, sources said, Montas opted against appealing the suspension and eventually accepted the standard first-time penalty for a banned substance.

Montas said in a statement issued by the MLB Players Association he was "deeply saddened" by the positive test.

"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," Montas said in the statement. "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 right-handed Montas has been the best pitcher for Oakland and was a legitimate All-Star candidate before the suspension. At 40-36, the A's are one game behind Boston for the second wild-card spot in the American League.

"The A's were disappointed upon learning of this suspension," the team 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."

Oakland does have a surplus of starting pitching primed to join the roster heading into the second half. Top prospect Jesus Luzardo is expected to make his next appearance at Triple-A after missing the season's first two months with a shoulder injury.

A.J. Puk, also a dynamic left-hander, is at High-A building up his pitch count after Tommy John surgery last spring. Sean Manaea, the ace of Oakland's staff last season, is nearing a rehab assignment following shoulder surgery in September.