He needed only 33 pitches through the first three innings, but allowed four runs on three hits while walking two in a 32-pitch fourth. The rookie took his second loss of the season and gave up five runs over 6 2/3 innings.
Manaea said his plan was to attack the Yankees' lineup on the inner-half of the plate. But he walked Jacoby Ellsbury to begin the fourth -- he said he opened the at-bat unable to locate his fastball on the outside part of the plate before trying to work Ellsbury back inside -- and the inning quickly fell apart.
"I tried going back inside and I missed that, too," said Manaea. "I just lost focus there. And then the next four batters I just wasn't as mentally sharp as I should have been."
The Yankees loaded the bases before Carlos Beltran singled in the first run of the game. Manaea responded by getting Chase Headley to fly out and Aaron Hicks to hit into a sacrifice fly, but then allowed a two-out two-RBI double to New York rookie Rob Refsnyder to make it 4-0.
Manaea said he didn't feel comfortable throwing his slider and thought the Yankees adjusted well and waited for his changeup.
"I have to have a third pitch. I can't just rely on my fastball and changeup," he said.
"I could tell early that he wasn't comfortable with that," A's catcher Matt McBride said of Manaea's slider. "And their lineup, with all those switch-hitters, has lot of right-handers. You don't want Sean, if he's not confident, to leave it up and have it stay up and get hammered. I think he wanted to stick with what worked last time and that was the fastball and changeup."
The turbulent start comes after Manaea's best outing of his career Monday, when he went 6 2/3 innings and gave up only one run in a win over the Rangers. McBride, who caught Manaea in that start and also caught him in the Minors, said the lefty didn't rely on his slider much in that start, either.
"I think he wanted to stick with what worked last time and that was the fastball and changeup. His changeup, when he's throwing it well, was really good," McBride said.
Mark Chiarelli is a reporter for MLB.com based in Oakland. This story was not subject to the approval of Major League Baseball or its clubs.