United's youngsters started the defensive of their Professional Development League title with a defeat away to Millwall on Saturday.
Three second goals condemned the Blades to defeat against the Lions, despite a late strike from substitute Sam Ackroyd.
The first-half was a pretty even affair, although the Blades did have the better of the chances, with Greaves coming closest.
But Millwall were quickly on the front foot after the break and opened the scoring when DUNCAN fired past Warhurst.
A second from ALEXANDER quickly followed and a third soon after from TIENSIA put the game beyond doubt.
United fought until the end though and were rewarded when ACKROYD, who had replaced trialist Swaine, headed home a Greaves free-kick from the right, but it proved to be nothing more than a consolation on a disappointing afternoon.
Lead Coach Derek Geary remarked: "It wasn't the best of games to be honest, they caught us with three quick goals, but there wasn't much in the game."
Blades: Warhurst, Weaver (Ompreon 45), Gordon, Belehouan, Foulstome, Broadbent, Hawkes, Potts, Mason (Capello 84), Greaves, Swaine (Ackroyd 70)
Unused: Harriott, Dewhurst.