A STOPPAGE-TIME penalty from Wayne Rooney broke Sheffield United hearts as below-par Manchester United secured a flattering passage into round four with a narrow 1-0 win.
Nigel Adkins side were full value for a replay at Bramall Lane, only for a dramatic denouement to see substitute Memphis - who injected impetus for the hosts in an otherwise poor collective display - felled by a sliding challenge by veteran midfielder Dean Hammond, who just caught the Dutchman with referee Jonathan Moss pointing to the spot.
Rooney duly sent George Long, who had little to do all evening, the wrong way for his ninth goal of the campaign, although the scarely-merited victory hardly eased the pressure on the under-fire Louis van Gaal.
The Blades were rarely troubled by the abject hosts, although they fatally cracked at the death.
Adkins named an unchanged line-up, while his counterpart van Gaal - not in keeping with many of his fellow top-flight managers - made just three changes from the side who beat Swansea last weekend.
Matteo Darmian, Cameron Borthwick-Jackson and Marouane Fellaini came in for Ashley Young, Phil Jones and Morgan Schneiderlin respectively.
Forty-six places may have separated the sides at the start of play, but you would have never have guessed, with a laboured first-half display from the hosts summing up their ordinary season in a nutshell.
The Blades were thoroughly comfortable, with the hosts failing to get behind them, with the lack of quality being distinct and the movement and speed of thought super-slow.
Long had little to do, aside from racing out of his goal twice to snub the danger when Rooney momentarily had a sniff of goal.
Aside from that, the young custodian did not have anything to do, with the Blades outfield players producing a diligent and spirited performance in front of him.
While the Blades did not produce any real threat at their other end, they did get into one or two promising situations, with the onus very much on the hosts to break them down - which they were wholly unable to do in the first period.
Rooney saw a late free-kick from a dangerous position blocked, while Daley Blind blazed a chance right on the interval wildly over the top following a quickly-taken free-kick, which sort of summed up their lame half.
One chance for the visitors arrived when Conor Sammon’s snapshot was tipped over by the alert David de Gea, only for play to be pulled back for an infringement.
The tepid home action did not get any better on the restart, although a moment of Blades danger almost yielded a chance, with Paul Coutts getting away from Bastian Schweinsteiger down the right, only for his pull-back to be cleared.
Van Gaal showing his hand first on the hour mark by bringing on Memphis and Jesse Lingard, with Anthony Martial - isolated on the left - moving to a central role and Rooney dropping back into midfield.
The frustration of home supporters was conveyed six minutes later when a rare piece of off-the-cuff endeavour ended in Memphis striking a powerfully struck shot wide, the prelude to sarcastic applause from the Old Trafford.
At the other end, Chris Basham saw a low shot saved after an encouraging run as the Blades sought to deepen the gloom for the hosts.
Darmian then broke momentarily into the box before his weak effort was gathered, with the hosts unable to generate a real head of steam, with Schweinsteiger dragging a lame effort wide on 75 minutes.
The onslaught never arrived, although Hammond had to be alert to impressively block a Martial effort nine minutes before time and not long after, Schweinsteiger received treatment after coming off second best in a challenge with Basham.
With time running out and United seemingly bereft of ideas, the ball found its way to Memphis on the left and he went down following Hammond’s injudicious challenge - with Rooney coolly slotting home the penalty gift.
Manchester United: De Gea; Darmian, Smalling, Blind, Borthwick-Jackson; Schweinsteiger, Fellaini (Pereira 78); Mata (Memphis 60), Herrera (Lingard 60), Martial; Rooney. Unused substitutes: Romero, McNair, Varela, Keane.
Sheffield United: Long; Brayford, Edgar, Collins, McEveley (Reed 78); Coutts, Hammond, Basham, Woolford; Sammon (Done 90), Sharp (Flynn 63). Unused substitutes: Howard, Baxte, Campbell-Ryce, Adams.
Referee: J Moss (West Yorkshire).