This, after he gave up 10 hits and nine runs over three-plus innings in his previous start at Anaheim.
"Just mostly inconsistent," said Stewart. "I felt like I threw some good pitches, and when I kept the ball down, it was pretty good. I just didn't do that consistently."
For the Red Sox, the loss was a milestone they wanted no part of it -- No. 90 on the season. This is the first Sox team to lose 90 games since the 1966 team went 72-90.
"We haven't had a good season," said Red Sox manager Bobby Valentine. "Every loss hurts -- all 90 of them."
There was a festive atmosphere at Camden Yards, with the Orioles on the verge of nailing down a postseason berth for the first time since 1997. The Red Sox didn't do anything to ruin the good vibes, getting swept in three straight.
"For me, it's definitely a good situation because it gets your adrenaline going and everything, but those guys, they're basically on the gas pedal right now," said Stewart. "They're doing a good job. That was one of those jobs where they beat us all series. They had their foot on our throat and pushed down at that point. I didn't feel like I was very consistent, and against a team like that, you can't be like that. You have to hit your spots, and you have to be consistent."
Stewart's troubles began immediately, as Nate McLouth led off the bottom of the first with a homer over the wall in center. The Orioles then loaded the bases with nobody out, and scored a second run on a 6-4-3 double-play ball off the bat of Matt Wieters. Jim Thome stung a single through the shift and into left-center, putting Boston in a 3-0 hole.
"After that, I'm just basically doing damage control to keep it at that score, but, obviously, I didn't do that," Stewart said. "That's basically all I thought of at that point."
After settling down in the second with a 1-2-3 inning, Stewart got in more trouble in the third. J.J. Hardy led off with a homer. Thome came through with another RBI single to make it 5-0, leading to Stewart's exit from the game.
"Zach was a little rusty, I guess," said Valentine. "First-inning jitters. Had the ball up a little. You know, I thought after the double play in the second inning, maybe he was going to get into a little groove down in the zone but Hardy hit a little ball, and before you know it, it was five runs. He'll have better days, that's for sure."
Cody Ross put Boston on the board in the fourth by banging his 22nd homer of the season.
Clayton Mortensen got the job done in long relief, as he's done so many times for Valentine.
The righty reeled off 3 1/3 innings, giving up a hit and a run while walking one and striking out four.
"I felt good out there," Mortensen said. "I was mixing pitches pretty well and just tried to throw strikes and keep us in the game."
Mortensen has made that a habit.
"I talked to Morty before the game. He's been terrific," said Valentine. "He's very competitive. He throws pitches that are tough to time. And in that situation, he's done a good job for us."
"I take that as a big compliment. I can say I'm pretty happy with the way my year has gone this year," Mortensen said. "At least I was able to show the front office what I can possibly do here, and hopefully I can be in the mix next year for whatever they have in mind for me. At least I got my foot in the door and said, 'Hey, I can pitch a little bit."'
Chris Davis did belt a solo shot against him in the fifth, making it 6-1, Baltimore.
Chris Carpenter and Vicente Padilla also turned in scoreless innings for the Red Sox.
Daniel Nava drew Boston closer in the sixth with a two-run homer, his fifth of the season.
"I thought we were back in it," Valentine said. "We were fighting the whole time. Hit a lot of balls hard today. Today was one of those days the hit column didn't show our at-bats, I thought balls were hit to the outfield hard and some at the infielders."
For the Red Sox, a disappointing season is down to just three more games, at Yankee Stadium, where the home team will be trying to win the American League East.
"Whether it's the Yankees or anybody else, we want to win," said Mortensen. "It would be great to be on their home field the last three games of the season and disappoint them and get them out of the playoffs. That would be awesome. We just want to get a win more than that. If so be it, they don't get to the playoffs because of that, even better."