Toronto scored its three eighth-inning runs on two walks, an error, a fielder's choice and Loewen's single to center to beat Daniel Bard (2-8).
With Boston ahead 3-2 going into the eighth, Edwin Encarnacion and Kelly Johnson both walked -- on Bard's first nine pitches -- and Bard then misplayed a bunt by Mark Teahen for an error that loaded the bases.
J.P. Arencibia grounded into a force out that got Encarnacion in and the Blue Jays tied the score. Loewen followed with a line drive into center that drove in two runs.
The Blue Jays opened the scoring with a pair of first-inning runs with Encarnacion and Kelly Johnson getting back-to-back RBI singles.
Boston tied the score in the second when Dustin Pedroia scored on an error and Kevin Youkilis scored on a ground out. The Red Sox took the 3-2 lead in the third when Jacoby Ellsbury tripled and Marco Scutaro hit a sacrifice fly. Adrian Gonzalez added a home run in the sixth that made the score 4-2. It was his 26th home run this year.