Boston loses for the eighth time in 10 games.
Jose Bautista hit two of Toronto's five home runs, and the Blue Jays beat the Boston 12-4 Sunday and sent the Red Sox to their eighth loss in 10 games.
In their first season under new manager John Farrell, the Red Sox had spurted to a 20-8 opening.
"I know it's a cliche, but things are evening out," Farrell said.
Ryan Dempster (2-4) gave up six runs and seven hits in five innings, his shortest start this season. The British Columbia native had been 6-0 against Toronto and Montreal in his big league career.
"They don't hand out trophies in April or May or June or July or August," he said in the middle of a quiet clubhouse as his teammates packed for three-city, nine-game trip. "We just have keep grinding out for the whole year, and at the end of the year see how it works out. The guys in here will continue to do that and bounce back and get on a winning streak."
Boston opened the season with its best start since 2002, but the past nearly two weeks have seen the club lose two closers to the disabled list, including Joel Hanrahan,to a season-ending forearm injury.
The Red Sox were a big league-best 18-8 in April.
"I think we've got a number of guys dealing with frustration right now. There's no question about it," Farrell said. "The key for us is maintaining our level of preparation and our work routine. Those are the two things that we can control. I know with the attitude of this group, we're going to continue to work, but we're getting tested right now. There's no question about it."