Matusz, who held Detroit to one run over five innings in his big league debut, was able to come out with the same devastating changeup against the Jays, but he struggled with his location.
"His changeup was OK," Trembley said, "[but] his fastball wasn't located as well as it was the first time he pitched. I think he's a four-pitch guy, and he got some fastballs in over the plate.
"He just didn't locate his fastballs, and at this level, they don't foul them straight back; they hit them, and that's what they did today."
The first fastball the Jays (53-57) took deep came off the bat of Kevin Millar, who drilled a solo shot to left field in the second inning to tie the game at 1.
"With Millar, he's got a quick bat, and I left a fastball out over the plate," Matusz said. "It was down, but it was down the middle, and he was able to hook it down the line pretty well."
The real damage came in the third inning, when Matusz (1-1) surrendered a leadoff hit followed by a home run to Marco Scutaro that plated two.
With two outs, Vernon Wells tattooed the first pitch he saw for another two-run shot. After a short conversation on the mound with pitching coach Rick Kranitz, Matusz tried to get Wells on a changeup down and away, but he missed.
"[It] was a changeup that was hung up," Matusz said. "I got a little bit tentative and tried to place it, rather than throwing it. It hung up there, and he took care of it."
Matusz allowed another single before Trembley pulled him. The manager did not want to allow the 22-year-old, who began the season in Class A and was pitching for Double-A Bowie before he was called up, to get hit too hard with so little experience under his belt.
"He was coming out of the game right there, because it wasn't his day and I'm going to do everything I can to help him," Trembley said. "He didn't locate his fastballs. It's obvious he wasn't able to locate it.
"You have to be real careful to make sure that you don't give these guys too much, too soon, too quick -- from the standpoint of there's no reason for him to stand out there [and] let them hit him all over the field. There's no reason for that."
Matusz left with the Orioles behind, 5-2.
The O's had some success against Halladay in the second and third innings, scoring a run in each, but the early deficit was too much to overcome.
"In a lot of cases, when you score early, you kind of take the wind out of the other team," Halladay said. "It's obviously harder to come back offensively when you're down. ... It's a mental grind for them after that.
"I know, from our side, obviously it's tough when you're in a hole and you have to come back. It's nice to put them in that situation."
The Orioles (46-65) plated another run in the sixth, coming up with three hits in the frame, including Melvin Mora's RBI single. Baltimore had nine hits and a walk against Halladay (12-5), but Toronto's ace kept baserunners from crossing home plate for the most part.
"We had some good at-bats, just Halladay, for some reason, he minimizes the damage," Orioles center fielder Adam Jones said. "We gave up a four-spot [in the third]. He scattered three runs over eight innings. If we were able to get that three- or four-spot, that could change the game. But he minimized our damage."
If the Orioles see Halladay again this season, it won't be until late September. By then, Matusz will have a few more starts behind him.
"Matusz will learn from it," Trembley said. "For goodness' sake, this kid is just getting his feet wet."
While Matusz was not pleased with Sunday's outing, he sees it as an opportunity for improvement.
"I'm upset about it, but I know what I did, and that's more important thing," he said. "I know what I did wrong. I know what I need to do to improve.
"I need to stop being so fine with my pitches and just attack the zone like I need to. Outings like this, you never want to have them, but the good thing about them is you're able to learn a lot."