Chris Heisey's throw home was late and Boesch slid in with the decisive run.
The score made a winner of Brayan Villarreal (2-1), who worked around a pair of walks in the seventh inning to record a scoreless frame in relief of starter Justin Verlander.
Fielder also socked a solo homer and Ramon Santiago went 2-for-4 with a bases-empty shot.
Reds starter Bronson Arroyo yielded two runs on five hits in a seven-inning effort, allowing no walks while fanning four.
Todd Frazier drove in both runs for National League Central-leading Cincinnati.