Mazzaro, a 22-year-old rookie, retired the next three batters, but the damage had been done. Aaron Hill hit a two-run homer, Alex Rios delivered a three-run double and the Blue Jays beat the A's, 7-2, on Sunday afternoon to take the series.
"I was struggling a bit," Mazzaro said. "They came out swinging the bats. I left the ball out over the plate. ... You can't miss your spots. They're going to hurt you if you do."
The Blue Jays added single runs against Mazzaro in the second and fifth innings. He allowed 10 hits and walked three in five innings.
Mazzaro threw 13 2/3 shutout innings over his first two starts, earning the win in both contests. He hasn't won a game since. Mazzaro has allowed at least six runs in each of his past four starts, and he's 0-3 with an 11.29 ERA in 18 1/3 innings over the stretch.
"We have to be careful to make sure we don't let him get down," Oakland manager Bob Geren said. "He came out flying, and now he's a bit down. We don't want him to lose any confidence.
"Being able to stay out there, that has to be a slight boost of confidence for him. He really could have gone in the tank there, and he didn't. That's something to build on."
Toronto starter Ricky Romero, meanwhile, went seven solid innings to earn his 10th win. He gave up two runs and eight hits, walked one and had five strikeouts.
Romero is 7-1 with a 3.36 ERA over his past nine starts.
"He's a pretty good pitcher," Geren said of Romero. "He made some good pitches -- that's for sure. ... They got him a nice lead out of the shoot. That's always an advantage for a pitcher."
Romero gave the A's some chances, but they were unable to capitalize. Oakland stranded runners in scoring position in the first, fifth, sixth and seventh innings.
Adam Lind went 3-for-4 and drove in a run, while Hill and Rod Barajas recorded two hits each.
Rajai Davis was 2-for-4 for Oakland, extending his hitting streak to a career-high-tying seven games. He's batting .400 over that span. Adam Kennedy and Mark Ellis pushed their hit streaks to nine and seven games, respectively, each going 1-for-4.
Russ Springer threw two innings of one-hit ball in relief of Mazzaro. It's the first time he's gone two innings in a game since Sept. 28, 2005.