Jonathan India and Eugenio Suárez homered for the Reds, NL wild-card contenders who have lost five of six.
Garneau’s first homer, a liner into the left-field seats, came during a six-run sixth inning in which Detroit had eight of its 18 hits.
Garneau, who had just one other long ball this season, got the first multihomer game of his career when he connected again the next inning.
Harold Castro drove in two with a second-inning double for Detroit, which tied a season high with 15 runs.
Vladimir Gutierrez (9-6) allowed four hits and four runs through 3 1/3 innings. The bullpen was worse, with Justin Wilson allowing three runs and three hits without recording an out, and Mychal Givens giving up the same while getting just one out.
Jeff Hoffman surrendered homers to Garneau and Grossman during a four-run Detroit seventh.
The 18 hits allowed by Reds pitching tied their season high.
Derek Holland (3-2) pitched a scoreless eighth and ninth to get the win.
Reds OF Jesse Winker (intercostal strain) is about a week away from progressing to a rehab stint at Triple-A Louisville. “I think he could benefit from at least a couple games in Triple-A,” manager David Bell said. … RHP Brad Brach (right shoulder) and RHP Art Warren (oblique strain) began rehab assignments at Louisville this week.
Tigers left-hander Matthew Boyd will be on the mound Saturday to face Reds right-hander Tyler Mahle. Boyd lasted four innings, allowing two runs and four hits in a loss to Toronto on Sunday. Mahle allowed four hits and two runs across seven innings but took the loss Sunday against Miami.
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