The Blades slipped to defeat at Bramall Lane for the first time since November, as Fleetwood left South Yorkshire with all three points on Tuesday night.
Goals from Conor McLaughlin and Devante Cole saw fourth-placed Town extend their unbeaten run in League One to 11 matches, as United struggled to find a way of breaching well organised opposition.
For the re-arranged clash against the Cod Army, McNulty was handed a start, whilst Lafferty returned to the starting XI having missed two games with a back problem. Done and Lavery were the ones to make way, with both dropping to the bench. New signings Hanson and O'Shea weren't registered in time to feature against Uwe Rosler's side.
Prior to kick-off, Hanson and O'Shea received warm applause from the home faithful as they were presented on the pitch.
Kicking towards the Kop in the first-half, the Blades enjoyed plenty of possession in the opening exchanges and Sharp's header from Freeman's cross forced Cairns into a save. In the next attack, Duffy linked up nicely with Lafferty and Sharp before firing over from a promising position.
In the 14th minute though it was the visitors who had the clearest sight of goal. Bell beat Basham in the air before Cole slipped in Burns, but his shot was deflected behind for a corner. But in the 20th minute, fortune certainly went the way of the visitors as they took the lead. The Blades surrender possession playing out from the back and from Grant's attempted effort on goal which was going harmlessly wide, the ball fell perfectly into the run of MCLAUGLIN who side-footed home past Moore.
Against well organised and resolute opponents, the Blades continued to enjoy plenty of possession, but despite a flurry of corners it was Fleetwood who should have doubled their advantage in the 37th minute. Dempsey advanced towards goal before slipping in Burns, but Moore was out quickly to make a smart save with his legs. Before the break, the lively Duffy tested Cairns with a free-kick after he had been fouled by Schwabl.
Boss Wilder made a double change at the interval, with Scougall and Lavery replacing Basham and McNulty. Following the changes, it became clear the Blades had changed formation and in the first attack of the second period Coutts saw an effort blocked from Sharp's lay-off, then Duffy fired over from the left.
United continued to force the issue and only a last-ditch block from Bolger denied Sharp, who had created space on the turn, the Blades skipper then robbed Bolger but his effort was collected easily by Cairns. Sharp was in the thick of the action and had another opportunity to shoot having got in behind Eastham.
Despite dominating possession and territory, the Blades fell further behind with 23 minutes remaining. Dempsey's hopeful ball forward saw COLE pressurise Ebanks-Landell into a mistake and he looped a bouncing ball over Moore and into the net. At the other end, Lafferty headed over from Freeman's cross, with Sharp appearing to be held back by Bolger.
The Blades refused to give in though, despite that blow, Sharp continued to look a threat, but despite Bolger's lack of discipline and several free-kicks in promising areas, Fleetwood remained tough to breach.
Three minutes of injury-time we signalled by the fourth official, but on what had become a frustrating night for United, the visitors held on without any alarms.
Blades: Moore, Fleck, O'Connell, Basham (Scougall 45), McNulty (Lavery 45), Sharp, Coutts, Freeman, Ebanks-Landell, Duffy, Lafferty (Done 89).
Unused: Ramsdale, Wright, Riley, Wilson.
Fleetwood: Cairns, McLaughlin, Bell, Eastham, Burns (Hunter 62), Grant, Bolger, Dempsey, Davies, Schwabl (Glendon 76), Cole (Ball 70).
Unused: Neal, Davis, Long, Maguire.