Diamond (3-2) baffled the Red Sox in a 96-pitch gem, yielding just three hits without a walk while striking out two batters.
The left-hander retired the final 15 batters he faced as the Twins evened their four-game road series at one victory apiece.
Boston starter Ryan Dempster (2-3) was touched for four runs (two earned) on five hits over seven full innings.
Jarrod Saltalamacchia provided the only run for the Red Sox with a ninth-inning homer.
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