Romero (8-11) lost his 10th straight decision, allowing five runs, seven hits and eight walks in 5 1-3 innings. He didn't strike out a batter.
Austin Jackson had three hits, and Scherzer (13-6) was sharp for the Tigers, allowing five hits, including Edwin Encarnacion's solo homer. He walked two.
Detroit needed three relievers to get through the eighth, when the Blue Jays scored two runs. Jose Valverde then pitched the ninth for his 24th save in 28 chances.
Valverde allowed two baserunners but eventually struck out Encarnacion to end it.
Romero walked six in the first two innings, allowing the game's first two runs on free passes with the bases loaded.
Romero is 0-10 with a 7.69 ERA in his last 11 starts. He was in trouble right away in the first inning on Tuesday when Jackson hit an infield single and went to second on third baseman Omar Vizquel's throwing error.
With two outs and a man on third, Romero walked Prince Fielder, Jhonny Peralta and Delmon Young to give Detroit the lead.
In the second, Romero walked Omar Infante with the bases loaded, and Miguel Cabrera followed with a double-play grounder that brought home another run to make it 3-0.
Encarnacion hit his 32nd homer of the year in the sixth, and at that point, Romero hadn't allowed a walk or a run since the second. But he couldn't keep it up, giving up a walk to Jeff Baker, a single to Andy Dirks and another walk to Gerald Laird to load the bases.
After Jackson hit into a force out at the plate, Infante hit a grounder to first baseman David Cooper, who hesitated and ended up not getting any out at all.
Romero left the game after that, and Cabrera added a sacrifice fly off Brad Lincoln to make it 5-1.
Toronto fought back in the eighth with an RBI double by Cooper and a run-scoring single by Yunel Escobar. Joaquin Benoit came on with two on and two outs and threw only one pitch, retiring Vizquel on a popup to end the threat.
NOTES: The last pitcher to walk at least eight batters with no strikeouts was Colorado's Greg Reynolds, who walked eight against Philadelphia on May 28, 2008, according to STATS, LLC. ... Scherzer has a career-high 186 strikeouts this year, surpassing his previous mark of 184 from 2010. ... Detroit RHP Anibal Sanchez (1-3) faces Toronto LHP Aaron Laffey (3-4) on Wednesday night.