A disappointing performance resulted in a narrow defeat for the Blades on Friday evening.
United had a chance to climb back into the top six but a second half goal from Nouha Dicko gave Hull City victory in front of the Sky cameras.
Fleck returned after suspension to take his place in midfield. He was one of two changes to the Blades team with Leonard also starting. Lundstram and Duffy dropped to the bench, where they were joined by Moore and Freeman, available again for the first time since October following a knee problem.
On a cold night, the game's first chance was created by the visitors but Blackman saved with his legs from Evandro and Dicko's follow up could only find the side-netting. United's first opportunity saw Fleck fire straight at goalkeeper McGregor.
Larsson attempted to deceive with a free-kick but was off target, as was Dicko after Evans was caught in possession. United's only other chances of note in a poor first half saw Baldock's shot saved and Fleck fire wide.
The start to the second half was better in terms of tempo. Sharp got the better of Aina but his shot was saved by the legs of McGregor and the home side took the lead soon after.
Blackman did well to push Dicko's shot around a post but the goal came from the resulting corner. The left-wing delivery saw the ball bounce around the Blades box before squirting out to the unmarked DICKO to beat Blackman from close range.
Hull went close again but Blackman was well-placed to collect Dicko's header from Aina's cross and for the final half hour the Blades were the more offensive side, although without seriously testing the home backline.
Stearman toppled in the area but no penalty was forthcoming and Lundstram headed wide a corner from fellow substitute Duffy.
Late on, Hull, expectedly, slowed down the action at every opportunity and kept the ball professionally to prevent United gaining any forward momentum and the additional minutes saw McGregor untested.
Hull: McGregor, Irvine, Dicko (Campbell 72), Diomande (Grosicki 64), Evandro (Stewart 77), Larsson, Bowen, Dawson, Clark, Aina, MacDonald.
Unused: Hector, Toral, Marshall, Tomori.
Blades: Blackman, Baldock, Stevens, Fleck, O'Connell, Basham (Donaldson 81), Clarke, Sharp, Leonard (Duffy 63), Stearman, L Evans (Lundstram 74).
Unused: Moore, Freeman, Lafferty, C Evans.