Both teams approached the game with two wins out of two in the competition, but the Reds proved too strong for the Italians with a comprehensive display.
Joshua Bohui and Mason Greenwood earned a two-goal lead and, although Hans Nicolussi Caviglia halved the deficit, James Garner added a third before the first half was out.
After Rafael Fonseca's sending off left Juve down to 10 men at the start of the second period, substitute Dylan Levitt added a 77th-minute fourth to ensure an impressive scoreline.
United were dominant from the outset with Bohui prominent down the left and sending in a dangerous cross after a one-two with Aliou Traore.