Left-back Tyler Blackett netted a stunning long-range strike in second-half injury time but it proved too little, too late after an uncharacteristically lacklustre performance from the Under-17s.
In front of a sizable crowd at the Showgounds in Coleraine, McGuinness' men struggled all evening to create clear-cut chances against a well-drilled Aspire outfit. The Qatar side also lacked penetration in the first period, but two quickfire goals from Jasper Uwaegbulam after the interval settled the contest.
Despite winning Monday's opening game 4-0, the defeat meant the young Reds finished fifth in the group and missed out on a spot in the quarter-finals. Instead, United will spend the rest of the week in Northern Ireland competing for the Slemish Trophy.
The Reds meet Co. Down at 3pm on Wednesday at The Warren, Portstewart.
United's Milk Cup squad: Liam Jacob, Bradley Anderson, Luke Giverin, Michele Fornasier, Sean McGinty, Thomas Thorpe, Thomas Lawrence, Larnell Cole, John Cofie, William Keane, Etzaz Hussain, Michael Keane, Joseph Coll, Jack Barmby, Donald Love, Charni Ekangamene, Tyler Blackett, Luke Hendrie.