United's U21s slipped to a disappointing defeat at the Canalside Sports Complex against a more experienced Huddersfield Town outfit on Tuesday afternoon.
The Blades named just one overage player, Marcus Williams, in their starting line-up, whilst the Terriers fielded four in the shape of Ian Bennett, Anthony Gerrard, Anton Robinson and Adam Hammill.
Despite a first-half strike from Danny Philliskirk, Town replied with four of their own, with Hammill, Murray Wallace, Matt Crookes and Connor Loftus all getting their names on the scoresheet.
The visitors fell behind as early as the fourth minute when WALLACE headed home at the near post from Sinnott's corner.
Wallace and fellow centre-half Gerrard went close to doubling the home side's advantage, before Long produced a fine save to keep out Hammill's long range drive.
United were struggling to deal with Town's set-pieces and from another inviting centre from Sinnott in the 20th minute CROOKES applied the final touch at the far post.
The Blades gave themselves a lifeline moments later when PHILLISKIRK headed home, but United couldn't find a leveller before the break.
Chris Morgan's youngsters started the second-half positively and only a top draw save from ex Blade favourite Bennett denied Matty Harriott an equaliser, the veteran clawing out the midfielder's goalbound drive from distance from under his crossbar.
The Terriers than regained their composure and pressed at the other end, Wallace went close from another set-piece before the experienced HAMMILL took centre stage, rifling home a 25-yard free-kick after a marauding run towards the penalty area.
With six minutes remaining Smith found substitute LOFTUS and he beat Long to make the game safe despite United's best efforts.
Blades: Long, Westlake, M. Williams (T. Williams 64), Harriott, Salt, Kennedy, Chapell (Smith 74), McFadzean (Pemberton 74), Ironside, Philliskirk, Tonne.