"The timing of it was great, especially after a short start yesterday," Athletics manager Bob Melvin said. "Guys know that they're going to have to pickup where Frankie left off. We had so many injuries last year in the rotation. Guys look forward to the opportunity, and it was Daniel's opportunity today."
Montas, 9-2 with a 2.70 ERA in 15 starts, was suspended 80 games last Friday for testing positive for a performance-enhancing substance.
Mengden (2-1) gave up four hits and struck out five for his first win since May 18 at Detroit.
"I know I belong here and I can pitch here so it's really tough to hear what happened to Frankie," Mengden said. "I've been called up and called down so many times, so I knew the time was going to come eventually to come back. I was always ready and hungry for the opportunity, got the opportunity, and tried to the best I could to keep the guys in it."
Yusmeiro Petit pitched the seventh, Joakim Soria the eighth and Liam Hendriks got the last three outs for his second save in four chances. Hendriks has not allowed a run in 23 of his last 25 appearances.
Oakland got its fifth shutout of the season -- first on the road -- and first since May 7 against Cincinnati.
Adam Wainwright (5-7) allowed two runs and seven hits in 6 2/3 innings with nine strikeouts.
"I had very good stuff today," Wainwright said. "Located the ball for the most part and stayed in good counts but they had a couple good swings on me 0-2."
St. Louis was shutout for the sixth time this season, and the third time in its last 14 games.
"We weren't able to put any consistent at bats together," Cardinals manager Mike Shildt said. "We're good and most offenses are good when you put together a lot of quality at bats and you string them together. We've had a stretch of that where we didn't get rewarded. Right now we've got to get back to just doing that, the quality at bats."
Taylor homered into the Cardinals' bullpen in right-center field in the fourth inning. It was his second home run in seven starts.
"I don't want to get up there and they just see the guy who gets called up and is going to be an easy out," Taylor said. "I don't want to be that at all. I kind of want people to try and remember me."
Chapman chased Wainwright from the game when he got his team-leading 19th homer into the Athletics' bullpen in left-center.
The start of the game was delayed 26 minutes due to rain.
Soria extended his record for most games by a Mexican-born pitcher to 675 a night after he passed Dennys Reyes to move into sole possession of the top spot. The team celebrated his accomplishment pregame.
STANLEY CUP IN THE HOUSE
The Stanley Cup Champion St. Louis Blues were honored in a pregame ceremony. Head coach Craig Berube and assistants Sean Ferrell, Mike Van Ryn and Steve Ott were on hand. Alex Pietrangelo, Pat Maroon, and Ryan O'Reilly paraded around the field with the Stanley Cup and visited with the Cardinals before the game.
BUSCH STADIUM ICON
Wainwright's start was the 300th of his career and his 184th game at the current Busch Stadium to tie Trevor Rosenthal for the stadium appearance record.
HICKS ON THE MEND
Cardinals RHP Jordan Hicks underwent successful Tommy John surgery to repair his torn ulnar collateral ligament. Cardinals general manager Michael Girsch said it is too early to put a specific timetable on Hicks' return. Hicks was 2-2 with a 3.14 ERA and a team-leading 14 saves in 29 appearances this season.
Athletics: RHP J.B. Wendelken was optioned to Triple-A Las Vegas after earning his first career win Tuesday night to make room for Mengden.
Cardinals: RHP Mike Mayers (right lat strain) and Ryan Helsley (right shoulder impingement) each made rehabilitation appearances for Triple-A Memphis. Mayers tossed a scoreless inning, and Helsley did not record an out allowing three walks and one hit against Nashville.
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