United's impressive run of Premier League wins came to a halt after a 2-0 defeat to high-flying Leicester City at the King Power Stadium.
The Blades fell behind to an Ayoze Pérez strike in the first-half, and despite an improved showing after the break, Demarai Gray added a second on the counter attack with 11 minutes remaining to deservedly seal the points for the home side.
Midfielder John Fleck returned to United's squad as Chris Wilder named an unchanged starting 11 following the home win over Chelsea.
Fleck came back from a recent injury to bolster the United bench, with Jack Rodwell missing out.
Both sides showed early intent with United almost creating a chance after two minutes. Ollie Norwood weighted a sublime pass in behind for George Baldock to latch onto, the pull-back looked good for David McGoldrick inside the area but James Justin made a terrific block as the striker pulled the trigger.
Leicester responded with Perez wasting a decent chance when he headed Luke Thomas' cross over. Moments later the hosts created a better opportunity when Justin's run down the right ended with a useful cross into the near post. Jamie Vardy's initial effort was blocked, but he was quick to get onto the loose ball when his low effort was saved by the boot of Dean Henderson.
The hosts enjoyed plenty of early possession but United initially looked solid in defence. On 23 minutes a good spell for the home side almost created a goal when Jonny Evans' deep cross was hit first time by Perez. Henderson blocked the initial effort and scampered after the loose ball to also smother the rebound.
Leicester's patient build up play was rewarded on 29 minutes with the game's opening goal. Justin came in off the right to find Perez who switched the play to Thomas. The youngster fed a pass back into Perez who had now advanced just inside the area, and quick as a flash he drilled a low shot beyond Henderson to open the scoring.
The Foxes remained in control of the half, but United showed signs of a revival when neat play between McGoldrick, Enda Stevens and Ben Osborn saw the latter deliver a teasing cross which had to be dealt with, whilst just before half-time Norwood's centre just eluded McGoldrick in the middle.
The Blades reacted to the first-half showing with a triple change as Fleck, John Lundstram and Lys Mousset were introduced for Osborn, Sander Berge and McGoldrick.
United initially had much more possession at the start of the second stanza but it was Leicester who had the next clear chance. Youri Tielemans played an inch perfect pass in behind for Vardy to chase and he looked a good bet to score as he closed in on goal but crashed his effort against the outside of Henderson's near post.
Just before the hour mark Leicester fluffed another chance when Ryan Bennett's ranging pass picked out Vardy whose touch saw Barnes clear on goal, but it was Henderson to the rescue for the Blades after another smart save with his feet.
Wilder made a fourth attacking change which saw Billy Sharp replace Chris Basham.
On 61 minutes it was Vardy teeing up Barnes again, with the outcome the same as Henderson denied the latter when again clean through on goal.
United received a shot in the arm when Wilder's side almost forced an equaliser on 63 minutes. Norwood delivered yet another dangerous corner which was met by a downward Egan header. The ball hit the stationery Wes Morgan forcing Kasper Schmeichel to make swift reaction save.
The Blades were now starting to make more of the running as Leicester made a defensive change bringing on Hamza Choudhury for Perez. Chances were at a premium though with Lundstram hitting a long-range strike which flew over the bar.
Leicester were now successfully playing on the counter and almost found a way through when Vardy teed up Tielemans from a tight angle, but he was denied by a last-ditch block from Egan.
That would prove to be a warning for what was to come as another swift counter brought Leicester's second goal. Evans stepped out from deep before releasing a terrific pass into the channel for Vardy. United were stretched after pushing bodies forward in search of an equaliser, with Vardy picking out sub Gray who finished well with a low shot into the bottom corner.
The hosts saw out the remaining 10 minutes as the Blades couldn't force a way through, with United looking to go again against Everton on Monday.
Leicester: Schmeichel, Bennett, Morgan, Evans, Justin, Ndidi, Tielemans, Thomas, Pérez (Choudhury 71'), Vardy, Barnes (Gray 76').
Unused: Mendy, Praet, Iheanacho, James, Hirst, Ward, Johnson.
Blades: Henderson, Baldock, Stevens, O'Connell, Basham (Sharp 59'), McBurnie, Egan, Norwood, McGoldrick (Mousset HT), Osborn (Lundstram HT), Berge (Fleck HT).
Unused: Jagielka, K. Freeman, Robinson, Moore, Zivkovic.
As it happened...
It's United to get this one underway.
Excellent early play from United.
Ollie Norwood weights a pass in behind for Baldock who cuts it back for McGoldrick to strike. James Justin to the rescue for Leicester who gets across for an important block.
Debutant Thomas wins a corner for Leicester - the first of the game.
Tielemans into the near post, Vardy tries to flick on but it comes to nothing.
Leicester come again and win another corner after a deflected Tielemans shot.
Tielemans delivers, helped on by Evans, cleared by United.
Quiet period in the game is ended by a chance for the hosts.
Young Thomas crosses from deep towards the back post where Perez heads over.
Big chance for Leicester.
Justin with a strong run down the right and a useful cross into the area. Vardy has his first effort blocked, but is quick to the second ball but he's denied from a tight angle by a smart save with his foot by Dean Henderson.
Sloppy from Leicester who concede a needless corner.
Norwood with a great delivery, but as McGoldrick attacks, Schmeichel punches clear under pressure.
Norwood free-kick is headed over by John Egan.
Barnes takes aim from distance but the strike hits his own player - Vardy - and deflects wide.
Good spell for Leicester.
Patient stuff from the hosts ends with Evans' cross which is volleyed towards goal by Perez - met by a stop from Henderson, who also smothers the rebound.
Even stuff so far. Leicester have had the better chances, but United looking solid.
Reward for the home side's patience.
Justin finds Perez who feeds it wide to Thomas, he plays it back to Perez just inside the area and he drives the ball low beyond Henderson.
McGoldrick, Stevens and Osborn combining as United respond.
The latter sends in a teasing cross which is dealt with, but that was better from the Blades.
Barnes is quick to see a run from Vardy but the execution on the pass is slightly too heavy and Henderson sprints off his line to gather.
Henderson claws away a Tielemans corner, and the return ball is headed clear by Basham as United stand firm.
Better from United.
A really good cross from Norwood is just too strong for McGoldrick.
Two added minutes.
Leicester 1 - 0 Blades
The Foxes are deservedly in front at the break after Ayoze Perez's strike.
Changes for United at half-time.
Fleck, Lundstram and Mousset on for Osborn, Berge and McGoldrick.
Leicester kick-off and we're back underway.
Early possession for United after half-time as the Blades look for a response.
Big chance for Leicester.
United have been better after the break but Leicester have the first chance of the half when Tielemans plays in Vardy but as he closed in on goal he crashed an effort against the outside of the post.
Another chance goes begging for Leicester.
Cross-field ball by Bennett picks out Vardy who plays Barnes clean through, but Henderson to the rescue with another save with his feet.
Fourth change for United as Billy Sharp replaces Basham.
United living dangerously.
Vardy again the provider as he sets Barnes clear again but once again he is foiled by Henderson.
Great chance for United this time.
A Norwood corner is headed down by Egan, it flicks off Wes Morgan and Schmeichel makes a reaction save.
United's chance has given the Blades a lift. Wilder's team have been better leading into the second break for drinks.
Leicester sub. Perez comes off and Choudhury comes on.
United now making most of the running.
Lundstram sails a long range effort over after some good build up play.
Dinked pass from Vardy gives Tielemans a shooting chance from a tight angle which is blocked by a last ditch challenge from Egan.
First yellow as Evans is booked for fouling Mousset.
Barnes is replaced by Demarai Gray.
That might seal it.
Leicester counter again. Evans with a terrific pass into the channel for Vardy who plays in sub Gray who fires a shot across Henderson into the bottom corner.
In the build up to the second goal, Egan was booked for a late challenge on Evans.
United win a corner.
Fleck delivers deep and O'Connell wins it but he's fouled the 'keeper. Free-kick.
Four added minutes.
Fleck into the book late on for a foul
Leicester 2 - 0 Blades
Goals from Perez and Gray seal the points for the Foxes.