United's U23s suffered a first home reversal of the season on a disappointing Monday afternoon at Bramall Lane.
Millwall's Kristian Brymora nets the only goal of the game early in the second-half as the Lions leave S2 with all three points, in addition to hitting the woodwork several times.
Two positives from the game were goalkeeper Simon Moore playing 90 minutes and making a string of fine saves on his return to competitive action, with skipper Ben Heneghan also playing a full game and producing an assured display.
The visitors almost drew first blood in the 14th minute as Twardeck forced Moore into a smart save at his near post. In response, Heneghan's header from Hallam's corner was headed off the line by Thompson.
Despite enjoying the majority of the ball, it was the Lions who continued to look more threatening, with Donovan hitting the post from Romeo's pull back. Then, McNamara got in behind the Blades backline, only for Moore to make a save with his legs.
Inside the opening 60 seconds of the second period, Hallam just failed to apply the finishing touch under pressure from goalkeeper King after Bennett's ball over the top hadn't been dealt with.
However, in the 54th minute it was Millwall who went in front, Twardek worked his way into the area and BRYMORA swept home with the aid of the woodwork.
Millwall hit the woodwork again moments later through Thompson, before Moore was needed again to deny Moore.
It was the visitors who looked more likely to score again and after the dangerous Twardek got the better of United's defence, his shot was deflected beyond Moore and bounced out off the bottom of the far post.
With five minutes remaining through, the Blades should have levelled, but Graham fired over after a goalmouth scramble wasn't cleared.
In injury-time the Lions hit the woodwork for a fifth time when Olaofe rattled the crossbar with the goal at his mercy. The final action of note saw substitute Slater red carded by referee Rhodes following a challenge on Rooney.
Blades: Moore, Bennett (Greaves 85), Thomas, Graham, Heneghan, Norrington-Davies, Gilmour, Doherty (Slater 66), Parkhouse, Carruthers (Cantrill 66), Hallam.
Unused: Ferguson, Dewhurst.