United's FA Cup exploits continue as League Two Plymouth Argyle were on the end of an impressive second half display.
Two second half penalties from Jose Baxter - after Plymouth had missed one - secured the third round place and Marc McNulty completed the victory, turning home a cross from Florent Cuvelier, who returned after an 11-month injury lay-off.
With McCarthy and O'Grady not allowed to play for the Blades in the FA Cup, chief Clough brought in Baxter and Higdon. There was a place on the bench for Belgian midfielder Cuvelier, who had been out of action since January with a cruciate knee problem.
Both sides probed early looking for an advantage but defences looked solid. For the visitors Reid hit a shot across goal and McEveley's timely tackle prevented Mellor making progress. At the other end in the opening 10 minutes, Doyle's 25-yard was wide and McCormick pushed Flynn's shot out of the reach of Higdon.
Flynn, operating at right-back once again, was back quickly to halt Kellett and Banton shot wide after he was handed a chance by Scougall's misplaced pass. Blizzard's 30-yard shot attracted the attention of Howard but was wide of the Kop goal.
As half-time approached Flynn again cut in from the right but shot wide and late on an Alessandra free-kick from the edge of the box was a yard over Howard's cross.
McNulty replaced Campbell-Ryce for the start of the second half and the substitute almost raced on to a long Howard clearance within 90 seconds. Three minutes later he clipped Baxter's pass just wide but in between these half-chances, Plymouth should have taken the lead. McEveley was adjudged to have fouled Alessandra but Reid wasted the penalty kick, firing wide.
Mellor's shot was blocked by Higdon before more drama as the clock ticked around to 55 minutes. Hartley was caught in possession and Purrington's first foul on McNulty was outside the box but he continued to pull on the substitute, who was eventually felled 15 yards from goal. Referee Sutton pointed to the penalty spot and a chat with one of his assistants reaffirmed his view. BAXTER took responsibility for the spot-kick and drilled low and hard past McCormick.
The penalty drama was not over. Seven minutes later and the Blades were awarded a second spot-kick, this time Nelson adjudged to have pulled back Murphy as he was about to shoot. This time referee Sutton relied on his assistant to make the decision but the outcome was the same, BAXTER scoring with a low and hard effort.
Blizzard's half-volley was wide as Plymouth looked to get back into the game and Flynn got back to prevent Reid shooting after McEveley lost possession.
Reed was introduced for Scougall and Baxter wasted a great hat-trick chance on 77 minutes. Flynn sprinted away from Reid and Murphy found Baxter in space but the subsequent shot, with just McCormick to beat, was rolled wide. Baxter also had another opportunity but rammed a free-kick into the wall in the final 10 minutes.
Baxter's chances of a hat-trick were curtailed when he was replaced with five minutes to go by Cuvelier - and both received a round of applause.
Late on McNulty robbed Cox and shot wide with Cuvelier in space but the subs did combine to crown the victory on 90 minutes. After linking up with Murphy, Cuvelier delivered a near post cross and McNULTY slotted past McCormick.
Injury-time was irrelevant as the Blades comfortably eased into the third round.
Blades: Howard, Harris, Basham, Flynn, Doyle, Murphy, Scougall (Reed 76), Baxter, McEveley, Campbell-Ryce (McNulty 45), Higdon.
Unused: McGahey, Turner, Kennedy, Khan, Cuvelier.
Argyle: McCormick, Mellor, Purrington (Norburn 71), Nelson, Alessandra, Banton (Cox 71), Reid, Blizzard (Harvey 80), Kellett, O'Connor, Hartley.
Unused: Bittner, Allen, Bentley.