Perez allowed six hits and three walks, striking out six, before turning it over to the bullpen. The A's were 1-for-10 with runners in scoring position against Perez and four relievers.
"We had our fair share of opportunities today," A's manager Bob Melvin said. "Those games are going to happen, but it doesn't mean you feel good about it. It's frustrating. They're working really hard. This group works as hard as any group I've been around. Just a tough game for us."
A's left-hander Sean Manaea struck out a career-high 10 batters in 5 1/3 innings, but he gave up six runs (five earned), despite allowing just three hits. Gallo hit a three-run home run in the second, and Manaea was charged with three more runs in the sixth. The Rangers loaded the bases when Nomar Mazara was hit by a pitch, Rougned Odor reached on a one-out error by first baseman Ryon Healy and Ryan Rua drew a walk. A's reliever Frankie Montas walked Robinson Chirinos to force in a run and gave up a two-run single to Gallo.
"Just the one mistake pitch," Manaea said. "Other than that, I felt like I pitched really well. I got a bunch of strikeouts on the slider today. That was really positive, and I felt like I was locating my fastball really well."
MOMENTS THAT MATTERED Gallo takes lefty deep: Gallo's home run off Manaea was his first against a Major League left-hander since taking Dodgers ace Clayton Kershaw deep on June 17, 2015. According to Statcast™, the ball had an exit velocity of 101 mph and a launch angle of 28 degrees for an estimated distance of 400 feet.
"It's nice to hit a home run in general, it just happened to come off a lefty," Gallo said. More >
Leclerc to the rescue: Rangers rookie right-hander Jose Leclerc entered with runners on second and third and one out in the sixth. The Rangers led, 3-0, at the time, and Leclerc kept the shutout intact by striking out Josh Phegley and getting pinch-hitter Matt Joyce to fly out to deep right.
"He doesn't seem to be fazed," manager Jeff Banister said.
"I think every pitch was working today ... inside, outside, changeup, everything." -- Perez, on his outing
Mazara was hit in the head by a 92-mph fastball from Manaea leading off the sixth. The pitch didn't hit him flush, but it was more than a glancing blow. Mazara was checked extensively by trainer Kevin Harmon, then stayed in the game. In the eighth inning, Rangers reliever Matt Bush hit A's outfielder Mark Canha in the hip with a pitch.
"He's OK," Banister said of Mazara. "He was down. We made sure Kevin checked him out, asked him a litany of questions, looked him in the eye, checked him out as he ran the bases. He's OK."
WHAT'S NEXT A's: The A's head to Kansas City, where the Royals will celebrate their home opener Monday at 1:15 p.m. PT. Right-hander Jharel Cotton, who allowed five runs in 4 1/3 innings against the Angels in his season debut last week, will start for the A's against Royals righty Ian Kennedy.
Rangers: Left-hander Cole Hamels leads the Rangers against the Angels at 9:07 p.m. CT in Anaheim on Tuesday. This is the opener of a three-game series and the start of a nine-game West Coast road trip against AL West opponents.