Hill hit a two-run homer and Toronto roughed up Mazzaro in a 7-2 victory over the Oakland Athletics on Sunday.
The Blue Jays scored five runs off Mazzaro before making an out. Adam Lind doubled after Hill's 26th homer, and Lyle Overbay and Vernon Wells each walked to load the bases. Alex Rios then hit a three-run double.
Romero (10-4) gave up two runs in seven innings, joining Tampa Bay's Jeff Niemann as the only rookies with double-digit victories. The left-hander lowered his ERA to 3.53 with his 12th quality start in 17 outings.
"He gives us an opportunity to win every single time," Blue Jays catcher Rod Barajas said. "For a young guy he has composure on the mound. When he does give up runs, he gets his focus back where it needs to be. He goes out and makes quality pitches. He's a competitor and he gives you his best effort every time."
Romero struck out five and walked one, improving to 7-1 with a 3.36 ERA in his last nine starts.
"You could see when he started struggling he just started throwing fastballs 93, 94, 95 mph," Toronto manager Cito Gaston said. "I think when you see that it shows you he has a lot of confidence. Sometimes guys get out there and get upset and can't throw strikes, no matter how hard you can throw."
Mazzaro (2-8), one of four rookies in the A's rotation, has lost eight consecutive decisions since winning his first two career starts. He gave up seven runs and 10 hits in five innings and has allowed at least six runs in four consecutive starts, the first A's starter to do that since Dave Stewart in 1991.
"It's been challenging," Mazzaro said. "They came out swinging. And I was leaving the ball over the plate. For one thing you can't miss your spots. They are going to hurt you if you miss your spots. If you are going to do that, you want to miss better. If you want to go in, you want to go in hard. And that's the thing I have to work on a lot. That's what I'm learning now."
Hill went deep for the second straight day and also singled in the second. He is batting .353 with six homers and 16 RBIs during a 12-game hitting streak.
"He's done a great job through hard work," Gaston said. "Like I keep telling you guys, I believe he's going to get a lot better."
Blue Jays SS Marco Scutaro was out of the lineup because of a bruised right heel. ... RHP Jason Frasor will handle the closing duties while LHP Scott Downs is on the disabled list. Downs was placed on the DL on Sunday after aggravating an injury to his left big toe on Saturday night. ... RHP Josh Roenicke, acquired from the Reds in the Scott Rolen deal, made his Toronto debut with a scoreless inning. Barajas was particularly impressed with his slider. ... A's manager Bob Geren said he had no update on the status of RHP Justin Duchscherer, who was scratched from Friday's scheduled rehab appearance for what the team called a "non-baseball issue." The two-time All-Star has missed the entire season rehabbing from elbow and back injuries.