For the second time this season, Chris Porter netted a late winning penalty at Colchester.
The striker had only been on the pitch a matter of moments but stepped up to net the winner on 87 minutes after a dominant performance.
Tuesday's game marked an impressive week for the Blades, who recorded nine points out of nine and chalked up three clean sheets.
Loanee Davies was handed his full debut after two substitute appearances, he took over from Flynn, who dropped to the bench after a hectic schedule of late - it was the only change to boss Clough's side following Saturday's victory over Bristol City.
Davies began on the right of midfield in a 4-4-2 system and the opening 20 minutes saw little to get excited about, with both sides attempting to figure each other out. From a Harris cross, a Baxter effort was deflected wide by Eastman and Murphy's shot, after cutting in from the left, was over as the Blades looked the more adventurous early on.
Howard looked untroubled as Sears hooked wide on 21 minutes but the Blades were soon back on the front foot. Murphy ran across the edge of the box and laid off to Coady, whose effort was blocked and Brayford's header from a Davies corner was saved. Harris capitalised on a defensive error but his effort was also blocked and then Baxter fired over after a quick turn.
Despite his protests, Doyle was booked for pulling back Morrison but the Blades remained the more prominent force as half-time approached. Davies almost got on the end of Murphy's cross and Scougall escaped the Colchester defence but his shot was pushed around a post by goalkeeper Walker.
Colchester made a double change at the start of the second half but the first chance after the break fell United's way - following Maguire's marauding run, Scougall's shot was deflected away. Murphy and Baxter switched positions and just after the hour the latter cut in from the left and curled against a post. Soon after Murphy eased around Sesay but fired over from an acute angle.
Goalkeeper Walker kept Colchester in the game on 66 minutes but how much he knew about it remains a mystery. Baxter found Brayford's run and his cross from the right was turned goalwards by Davies but the ball hit the sprawling home stopper.
Collins sidefooted over a Davies corner and Baxter sliced wide as United's momentum continued - full-backs Brayford and Harris were regularly adding themselves to attacks.
Flynn replaced Murphy as Clough made his first change with less than ten minutes to go and Doyle fired over after bursting through an Eastmond challenge. Porter was the next introduction, taking over from Baxter, and it was decisive.
From United's next attack, Sears was adjudged to have clipped Davies inside the box and referee Scott immediately pointed to the penalty spot. At the same end as his FA Cup winner back in November, PORTER stepped up and comfortably slotted home in front of the Unitedites.
Winning the penalty was Davies' last contribution, he was replaced by Hill, and five minutes of added time was announced, the only threat coming from an Eastmond cross but Ibehre's header was wide and moments later the final whistle signalled a long but happy trip home for the 438 Unitedites who made the trip to Essex.
Colchester: Walker, Wright, Dickson, Okuonghae, Morrison (Eastmond 45), Sears, Bean, Ibehre, Eastman, Wilson (Sesay 45), Vose.
Unused: Cousins, Massey, Sanderson, Olumfemi, Bonne.
Blades: Howard, Harris, Brayford, Maguire, Doyle, Baxter (Porter 84), Collins, Coady, Scougall, Davies (Hill 88), Murphy (Flynn 81).
Unused: Miller, Paynter, Long, Kennedy.