Tigers starter Justin Verlander had blanked the Red Sox on two hits over 6 1/3 innings when Napoli powered a pitch over the left-center field fence, breaking a scoreless tie.
John Lackey (1-0) scattered four hits over 6 2/3 innings to post the decision, issuing no walks and striking out eight as the Red Sox posted a second straight American League Championship Series victory.
Verlander (0-1) finished with 10 strikeouts over eight innings, allowing four hits with one walk.
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