Even goalkeeper Tim Howard hesitated on his line for just long enough on the deciding goal to allow Oscar Boniek Garcia to slip the ball under him onto Bengtson's sprinting feet.
Bengston, who plays for Major League Soccer's New England Revolution, scored the winner when defender Geoff Cameron and goalkeeper Tim Howard both went for a pass by Maynor Figueroa to Oscar Boniek Garcia.
Oscar Boniek Garcia's corner kick went all the way to the opposite side of the field, bounced four times and was played back to the center by Victor Bernandez after no American chased the ball down.
Two goals from oscar Cardozo and one apiece from Pablo Aimar and Javi Garcia ensured Benfica's smooth progress, where they will now face either Marseille or FC Copenhagen.