United's Under 18s bowed out of the FA Youth Cup with a 3-0 defeat away at Derby County.
First half goals from Jason Knight and Louie Sibley put the hosts in control at Pride Park before a penalty from Tyree Wilson in the early minutes of the second half put the game to bed.
Looking to build on an impressive victory at Middlesbrough in the previous round, the Blades found themselves up against it throughout against a very impressive Derby side.
After doing most of the pressuring early on, Derby took the lead after just 11 minutes when Knight got free in the box and powered a header past Marcus Dewhurst after Sibley's corner found him.
United spent the next half an hour looking for a foothold in the match and kept Derby at bay for the most part with some solid defending.
Whittaker should have doubled the lead for the hosts when he got free inside the box but Dewhurst did brilliantly to block his close-range effort when the striker had the whole goal to aim at.
But just before the break the hosts did double their lead and it came at a tough time for a United side who had battled throughout the half.
Some good work from Wilson down the left saw the left-back find Sibley, the striker taking a touch before hammering an effort into the roof of the net.
It left the Blades will a mountain to climb after the break and despite the introduction of Nickseon Gomis, that task was made virtually impossible when United conceded a penalty less than minutes into the second half, Wilson stepping up to thump an effort into the corner.
The Blades dug deep to find a way back into the game but couldn't create any clear chances and it was left to Dewhurst to keep impressing at the other end as Derby found more gaps as the Blades pushed forward looking for a way back into the game.
The Blades goalkeeper saved in a one-on-one situation from Sibley and repeated the feat in stoppage time as United's cup run came to an end in Derby.
Blades: Dewhurst, Williams, Boyes, Neal, Gordon, Belehouan, J. Kelly (Gomis 46), Broadbent, Ompreon (Potts 62), Brunt, Gaxha (Ackroyd 62).
Unused: S. Kelly, Viggars.