We fell to our second defeat of the season after a lacklustre Monday night display at Bramall Lane.
Lys Mousset scored the only goal of the game, tucking home from close range after a corner was headed down at the far post.
We struggled to create many chances after falling behind, with the well-organised United defence frustrating our attacking trio of Nicolas Pepe, Pierre-Emerick Aubameyang and Bukayo Saka. Even Alexandre Lacazette's late introduction on his return from injury was to no avail.
The Blades have the joint best defensive record in the Premier League with leaders Liverpool, and on this evidence you could see why.
But it could have been difference had Pepe converted our best chance of the first-half. He won the ball himself in the right back position to spring a rapid counter attack, and Sead Kolasinac squared the ball for the Ivorian, but he failed to make proper contact when in front of goal.
We were punished 10 minutes later when the hosts made one of their many first-half corners count. Jack O'Connell outjumped Matteo Guendouzi at the far post, and headed down for Mousset to tap home from two yards.
We just weren't creating enough, and now United had the lead, they were proving even more difficult to break down.
It looked as Bukayo Saka could bring us back into the game when he burst into the box after swapping passes with Joe Willock. He went down after a challenge from John Egan, but rather than award the penalty, Mike Dean booked Saka for diving. It seemed a harsh decision, as there appeared to be some contact, and assistant coach Freddie Ljungberg was also shown a yellow card for his protests.
Pepe had a free-kick saved in the second-half, but for the first time this season we failed to find the target and left Yorkshire pointless.
HOW IT HAPPENED
WHAT IT MEANS
Our next game is just three days away, back in the Europa League, at home to Portuguese side Vitoria, kicking off at 8pm.
Our next Premier League game is also at home, against Crystal Palace this Sunday at 4.30pm.
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