Cahill, who fell to 6-9 with his fourth loss in five decisions, allowed six runs on 10 hits and two walks over five innings.
"The whole game was like the first inning last time," Cahill said. "I couldn't get into a groove where I was pounding the zone."
Against the Twins, Cahill walked the game's leadoff batter and fell behind three of the four batters he faced in the first frame. On Monday, he threw a first-pitch ball to 14 of the 25 hitters he faced.
Contrast that with the work of Red Sox ace Josh Beckett, whose first 11 pitches of the game were strikes. He was ahead of hitters all night, and that's about all you need to know about Oakland's 27th loss in 42 games.
"He had an 80 percent [strikes] ratio going into the seventh," manager Bob Geren said of Beckett, who struck out 10, walked one and gave up eight hits while pitching into the eighth. "It was a really good lesson for our young pitchers. ... He's obviously one of the better pitchers in the league."
Sitting in the dugout while Beckett worked his magic, Cahill was able to appreciate his counterpart even while his own night was going to the dogs.
"I think through the first six innings, he threw 12 balls," Cahill said, coming up three shy of the actual count. "He was forcing guys to swing the bat. That's what I was trying to do."
"We had a game plan and stuck to it," said Beckett, who picked up his 12th win and credited catcher Jason Varitek for battling through a sore hand, sore legs and a sore shoulder. "I'm amazed at what he does."
Pedroia padded the lead with a sacrifice fly after Jacoby Ellsbury's leadoff triple in the third, and the Red Sox broke open the game an inning later, getting an RBI double from new acquisition Adam LaRoche and RBI singles by Varitek and Ellsbury.
Jason Bay added an RBI single in the fifth before the A's finally broke through against Beckett on Kurt Suzuki's two-out RBI single in the sixth.
Eric Patterson's sacrifice fly in the seventh made it 6-2, but Boston got those runs right back when a two-out popup behind third base by Jed Lowrie barely eluded the glove of sliding shortstop Orlando Cabrera and bounced into the stands for a two-run double.
"We'll take runs any way we can get them," Red Sox skipper Terry Francona said.
Beckett (12-4) was lifted after Scott Hairston's leadoff triple, after which Suzuki picked up his second RBI of the night with a groundout.
"We've actually been swinging the bats pretty well the last few days," said Geren, whose club got two hits each from Ryan Sweeney and Adam Kennedy. "It came together a little late."
Five games into a daunting road trip through New York and Beantown, the A's have seen a full turn from their young starting rotation, which features four rookies -- all under the age of 24 -- and an 25-year-old "ace" in his first full season in the Majors.
On Thursday in New York, 22-year-old righty Vin Mazzaro was good early, then bad in a hurry. On Friday, 21-year-old lefty Brett Anderson was just OK. Gio Gonzalez, a 23-year-old lefty, was good on Saturday, and lefty "veteran" Dallas Braden was bad on Sunday.
"It's been all over the place," conceded Geren.