Baltimore scored four runs in the first inning but couldn't knock San Diego starter Jon Garland from the game. Instead, it was Arrieta who had the abbreviated and ineffective outing, resulting in the Orioles' Major League-leading 50th loss, a 9-4 defeat on Sunday in front of 28,029 at PETCO Park.
"I just didn't hold up my end of the bargain," said Arrieta, who gave a run in the first and second innings before the Padres generated a four-run third.
"I felt like I could still do a good job of pitching deep into the game and give the team a chance to win. Unfortunately today, I just didn't have it."
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Looking to become the first Orioles rookie to win his first three Major League starts, the 24-year-old Arrieta (2-1) battled from the beginning, consistently working from behind in the count, and was removed after just three innings of work. His final line of six runs -- five earned -- on five hits and three walks forced his ERA up to 5.06 in a disappointing 56-pitch outing.
"His command wasn't there with any of his pitches," said interim manager Juan Samuel. "He threw a few good changeups, but mainly you have to work with the fastball so your secondary pitches could work. And he just couldn't get it going."
Arrieta had been backed by superb defense in each of his first two outings, and he was helped again on Sunday as Luke Scott made a great headfirst catch in foul territory, diving over the wall to retire Scott Hairston. But Scott's error at first base in the following frame allowed Will Venable to score and the Padres to stick around.
San Diego plated another quick score in the third before Venable sent the first pitch he saw from Arrieta (2-1) into the right-field stands for a decisive three-run homer. The hanging change was a pitch selection Arrieta later questioned, but catcher Matt Wieters defended.
"I think the pitch was fine," Wieters said. "[Venable is] an aggressive guy, so we want to go offspeed there. I just think the location was not great. He was probably a little out in front, but it was high enough and middle enough of the plate where he could hit it out."
Wieters snapped an 0-for-16 skid with a two-out bases-clearing double off Garland in the first inning, helping to quell some of the concerns regarding his slow development. The first-inning line drive was Wieters' first three-RBI game since May 1 against Boston.
"I just relaxed, cleared the head and tried to hit the ball hard," Wieters said of his fourth hit in 11 games. "I was able to get that one to fall in and I just missed a couple of more. I felt much better at the plate [on Sunday]."
The O's came out swinging against Garland, capitalizing on a defensive miscue -- which was later ruled a hit -- for an early cushion. After Corey Patterson and Nick Markakis singled, Scott worked a two-out walk and Adam Jones hit a routine ground ball to third base. But Lance Zawadzki double-clutched the ball, struggling to get it out of his glove, and allowed a hustling Jones to beat the throw. Patterson scored on the play and Wieters cleared the bases with his shot in the left-center-field gap.
Baltimore looked poised to tack on to that lead in the second, as Arrieta worked an eight-pitch walk and Patterson singled to put a pair of men aboard. But Miguel Tejada swung at a 2-0 offering and Zawadzki turned an impressive double play to squelch any thought of the O's chasing Garland from the game.
"If that balls goes by him, it's probably going to be two runs and a double for me," Tejada said.
Added Samuel: "We definitely would have added some more runs. [Garland] probably would have been a different pitcher."
Instead, Garland effectively shut the door. After throwing 51 pitches in the first two innings, he went on to last six frames, retiring the final 14 batters he faced.
"The big key was throwing my off-speed pitches for strikes," Garland said. "Some of the swings that they were taking when I did throw the fastball, they were late on some pitches, and I think that's going off of throwing that off-speed stuff for strikes."
Garland (7-5) allowed just one hit after the first inning, en route to becoming the 54th opposing starter to last six innings against Baltimore. The result was the largest blown lead for the Orioles all season, a loss that drops the O's to 19-50 and concludes a 2-4 West Coast road trip.
"That's why this is a tough ballgame," said Samuel, who dropped to 4-11 as the Orioles' manager and is still searching for his first series win.
"You think, 'OK we have a shot to win two out of three'. We were in a good, good position to win and they come back. You figured if we scored four runs, we need to add a few to win, and they just got tougher."
Brittany Ghiroli is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.