Buchholz (2-9) allowed five runs, three hits and three walks in 2 1/3 innings, the shortest start of his career. The 24-year-old, who threw a no-hitter against the Orioles last September, is 0-7 in nine starts since May 2.
After a game in which only 30 of his 60 pitches were for strikes, Buchholz was optioned to Double-A Portland.
"People have tough games. I've had a tough season," said Buchholz, who spent two months in Triple-A from mid-May to July 11. "Down there is where you develop. Up here it's a pennant race. ... If you ask if the decision was right, I'd say it was."
Buchholz was given an early 4-0 lead but couldn't make it stand up. He has a 6.75 ERA and opponents are batting .299 against him.
"Tough situation. We're in a pennant race and we're in Boston and a lot of focus is on him and he feels the responsibility," Red Sox manager Terry Francona said. "I think coming into this game we were all excited about watching him pitch, but the way it unraveled makes it kind of obvious we had to do something."
Buchholz started with a 1-2-3 inning, but gave up three runs in the second and was yanked after hitting a batter and walking another in the third.
"Instead of going out there and gritting my teeth and say I'm going to get this guy out, I go out there and think, `What's going to happen next?"' he said. "That's not a very high confidence level."
Down 4-0 in the second, Baltimore scored 10 straight runs against Buchholz and two relievers. Hernandez put the Orioles up 6-4 with a third-inning drive, and Mora's 21st homer of the season made it 10-4 in the fourth.
"Against Boston you've got to be able to hit with men on base," Orioles manager Dave Trembley said. "You've got to score when you get opportunities to score because you never know if you're going to have enough runs."
Mora added a fifth-inning sacrifice fly to boost his team-high RBI total to 95. He has 10 homers and 47 RBI in 31 games since the All-Star break.
Chris Waters (2-0), making his fourth major league start, gave up five runs, four earned, and eight hits in five innings for Baltimore.
"It was tough early but he kept us in the game, hung on right there and he gave us a chance to come back," said Hernandez, Waters' batterymate. "He kind of figured it out a little bit. He went through a rough start but he ended up pretty good."
Jason Bay homered for the Red Sox, his fourth in 17 games since being traded from Pittsburgh.
Boston got three hits and two walks in the first inning but could muster only two runs. Dustin Pedroia hit a one-out triple and scored on a single by Kevin Youkilis before Jed Lowrie walked with the bases loaded. Jeff Bailey then struck out and Coco Crisp hit a grounder in the hole that shortstop Juan Castro turned into a force at third.
A baserunning error cost Boston a scoring chance in the third. After Lowrie hit a leadoff double, he took a few steps toward third as right fielder Nick Markakis caught a fly ball. Before Lowrie could get back to second, Markakis threw him out.
"That's another momentum changer for us," Trembley said.
The Orioles went up 7-4 in the bottom half. After Buchholz was lifted, Hernandez greeted David Aardsma with a drive to center that Crisp failed to grab with a leap over the 7-foot wall. Montanez walked and Castro followed with an RBI single.
Mora connected in the fourth off Javier Lopez, and Bay went deep in the fifth to make it 10-5.
- After the game, the Orioles optioned LHP Garrett Olson to Triple-A Norfolk. Baltimore will recall a pitcher from Norfolk to start Friday against the Yankees, Trembley said.
- Timlin made his 1,049th career appearance, one short of Kent Tekulve's major league record for right-handed relievers.
- SS Julio Lugo, who has been on the DL since July 12 with a strained left quadriceps, suffered a setback Wednesday, Francona said.