The contest was stopped for a number of minutes while paramedics attended to the fan in question, before the man was taken to hospital. The condition of the supporter is not yet known.
The hosts needed just 10 minutes to edge ahead in the contest, with Raphael producing a thumping header past his own goalkeeper from a Grant McCann corner, before Vassell doubled the lead soon after to send United on their way.
Nevertheless, Peterborough's route to Wembley in the final of the competition became slightly more complicated after the half-hour mark when Ranger reduced the arrears, racing through on goal before curling past goalkeeper Robert Olejnik.
McEveley was given his marching orders shortly after for connecting with the face of Britt Assombalonga with a high boot but, despite the numerical disadvantage, the Robins pulled level on the stroke of half time courtesy of Brisley's own goal.
Play was resumed after the incident involving the supporter as the spoils were ultimately shared.