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The 27-year-old lost the sight in his left eye in an on-pitch incident seven years ago but recovered to return to the game and qualify for Italy through residency after joining Italian side Benetton.
McKinley came off the bench as a 61st-minute replacement for Carlo Canna and kicked a penalty to help edge Italy to victory. Canna had earlier struck three penalties and converted Simone Ferrari's early try.
The visitors, for whom Ben Volavola kicked a penalty, went in level at 10-10 at halftime through Leone Nakarawa's converted try, but Conor O'Shea's side edged ahead after the interval to avenge their summer defeat by the Fijians.