United advanced through to the last 16 of the Emirates FA Cup with victory over Preston North End at Bramall Lane.
In a game of few clear cut chances, Billy Sharp's late penalty in front of the Kop proved decisive.
For the fourth-round tie, Heneghan was handed his debut, with Ched Evans returning amongst the substitutes for the first time since ankle surgery. There were a total of seven changes, with Leonard, Lee Evans and Holmes all cup-tied.
The first action of note came in the fourth minute, with Welsh lucky to only see yellow for a reckless and dangerous lunge from behind on a turning Duffy.
United had shaded a scrappy opening 15 minutes, before Donaldson fired wide at the near post after a cross from the overlapping Baldock. Donaldson then nearly forced Fisher into a speculator own-goal, before Stearman fired over from the resulting corner.
As the half hour ticked by, referee Scott again had his yellow out of his pocket, this time for Lafferty, for a mistimed tackle on Fisher.
The Blades ended the half positively, with two corners in succession, but despite having numbers there, Preston defended them well.
United were quickly out of the blocks in the second-half and Donaldson came within a matter of inches of turning Duffy's near post, only to see the effort hit the side netting.
In the 52nd minute, the deadlock was almost broken in front of the Kop, only for Rudd to deny Lafferty from point-blank range from another right-wing centre from Duffy. Lundstram was then booked, despite appearing to get some of the ball, when challenging Horgan.
As the hour mark approached, Wilder made two positive changes, with Basham and Evans replacing Heneghan and Donaldson. Preston introduced Hugill from the bench, before Johnson was booked for a lunge on Fleck.
Preston's first serious threat on United's goal came with 12 minutes remaining, Johnson engineered time and space on the edge of the box and his effort was inches wide with Moore scrambling across his line.
But within a couple of minutes the Blades were in front from the penalty spot. Basham's pinpoint ball forward found SHARP and Clarke was adjudged to have not played the ball when sliding in to block. From twelve yards, the United skipper confidently fired past Rudd.
Four minutes of added time were signalled and after an earlier challenge on Moult, the goalkeeper was in the right place to deny Hugill from six yards out and that was enough to see of the challenge of North End.
Blades: Moore, Baldock, Fleck, Lundstram, Sharp (Stevens 90), Donaldson (Evans 59), Wright, Stearman, Duffy, Heneghan (Basham 59), Lafferty.
Unused: O'Connell, Wilson, Eastwood, Carruthers.
Preston: Rudd, Clarke, Horgan (Barkhuizen 59), Browne, Johnson, Fisher (Woods 68), Welsh (Hugill 68), Moult, Huntington, Earl, Bodin.
Unused: Gallagher, Spurr, Maxwell, Robinson.