Porcello, 20, was 1-0 with a 2.21 ERA over 20 1/3 innings in four August starts before Sunday. Perry, 22, had a 2.30 ERA in 12 relief appearances since being recalled on July 18.
On Sunday, both rookies hit a wall.
Each surrendered a pair of home runs -- a three-run homer and a solo shot apiece -- as the Tigers dropped their rubber match with the A's, 9-4.
Detroit maintains its 2 1/2-game lead in the American League Central standings, as the White Sox lost a rubber match of their own against Baltimore.
"Those home runs -- that's what hurt us," said Porcello. "Take those away and it's a different game."
There was no taking these homers away, though. Jack Cust took Porcello deep twice. Porcello allowed eight hits over 5 1/3 innings as he fell to 10-8.
"I just fell behind him," Porcello said. "The first one, I got behind with a slider and changeup out of the zone. Then I left a fastball right over the plate and he jumped on it. ... The second one, down 3-1 [in the count], it was pretty much the same story. He just turned on it."
"We've faced him three times this year," Cust said. "The more times you face a guy, the more familiar you get. I was looking for good pitches and happened to get a few from him today,"
Curtis Granderson and Miguel Cabrera homered for the Tigers. Granderson's leadoff shot hit the right-field foul pole on the fourth pitch of the game.
Cabrera took Michael Wuertz out in the eighth, notching the 200th homer of his career, to extend his hitting streak to eight games. He's batting .531 over that span.
The loss handed the Tigers their 10th straight loss of a road series.
The Tigers haven't won a road series since taking three of five from the White Sox at U.S. Cellular Field in early June.
"I don't think [playing on the road] has anything to do with anything," manager Jim Leyland said. "We haven't played well. It's as simple as that. ... [Oakland is] not having a great year and they just kicked our [behind] two out of three. You have to respect everybody. It's the Major Leagues."
Oakland put a four-spot on the board in the third. Cliff Pennington singled, then tagged and took second on a fly to Granderson in center. Rajai Davis scored Pennington with a soft single to right.
After walking Ryan Sweeney, then striking out Scott Hairston, Porcello served up Cust's three-run shot.
"[Porcello] couldn't keep the ball in the ballpark," Leyland said. "Other than that, he did pretty good. ... Solo home runs are one thing. Three-run home runs are something else."
Perry allowed both of his homers in the eighth. Landon Powell, the first batter he faced, hit the three-run homer, then Pennington homered two batters later.
"I was a little disappointed with him," Leyland said of Perry. "I was looking for the strikeout. ... He didn't bounce back too good today [after pitching yesterday]. We'll watch that."
With two on and two out in the seventh, Leyland pinch-hit Marcus Thames for Adam Everett. It worked. Thames brought Cabrera home with a single, moving Brandon Inge to second. Ramon Santiago then singled to load the bases.
Leyland played the pinch-hitter card again, subbing Ryan Raburn for Granderson. He didn't have the same luck, though, as Raburn flew to center to leave them loaded.
"Raburn put a good swing on his," Leyland said. "He had a very good at-bat. Thames got one with his. ... We just couldn't find a gap."
Adam Loberstein is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.