But the Dutch boss is in a huge battle to still be in charge of the Reds when the trophy will be lifted at Wembley in May.
The pressure is growing on the United manager after another uninspiring performance, with United legend Paul Scholes acting as spokesman for the fans when he dismissed his old club as ‘boring’.
So dreary and dull were Van Gaal’s side, that his own supporters gave an ironic standing ovation to Memphis Depay’s 68th-minute strike that drifted two yards off-target, simply because it was a small glimpse of entertainment.
And the reaction was repeated two minutes later when Matteo Darmian stabbed a weak effort at George Long’s goal for United’s first attempt of the game on target.
Van Gaal admitted his side weren’t good but the statistics are damning against his insistence that United do play attacking football.
This was the 10th consecutive game that United had failed to score before half-time at Old Trafford, the 10th time this season that United have scored one or no goals at home and would have been their eighth goalless draw of the season until Rooney’s 93rd-minute strike.
“In a cup game, the most important thing is always to win, and at least we did that,” said Van Gaal.
“We are in the next round and that is all that really matters, but I could not be so happy with the performance.
“We moved the ball too slowly and we needed to play with wingers to make the pitch as big as possible.”
Not since the late 80s have United seemed so far beyond their rivals. Back then, Alex Ferguson was scratching around for results when the FA Cup to came his rescue.
Mark Robins famously scored a winner at Nottingham Forest in the third round in 1990, United went on to win the trophy and so began the biggest dynasty the English game has ever seen.
But even the staunchest supporter could not see Saturday’s result heralding an exciting new era at Old Trafford.
United were ponderous and ploddy, and lacked a cutting edge against the organised Blades.
Anthony Martial was again anonymous — stuck out on the wing for the first hour, Wayne Rooney battled away with little opportunities to feed on while Juan Mata and Andreas Herrera struggled to create. Depay at least added some spark when he came on after the hour and won the penalty when he drew a reckless challenge from Dean Hammond.
Van Gaal again needlessly started with a defensive shield of Bastain Schweinsteiger and Marouane Fellaini against a side that began with two strikers but quickly reverted to just Billy Sharp in an attacking position.
And surprisingly, he praised his side for stopping the visitors from creating any opportunities.
“Sheffield United did not create any chances either, but no one talks about that,” he said.
“They played with 10 men behind the ball and that is the most difficult situation to overcome in football, it is easier to defend than it is to attack.”
United’s executive vice-chairman Ed Woodward, the man with his finger on the trigger, was seen puffing out his cheeks after Rooney’s late winner.
He has backed Van Gaal but is difficult to see the veteran continuing at the end of the season, particularly if United miss out on a place in the Champions League.
That makes it a big week for the Reds, who are three points off the top four, and face tricky trips to Newcastle on Tuesday and Liverpool on Sunday.
A further concern is they could be without the experienced Schweinsteiger for both games after he twisted his right knee in a full-blooded tackle with Chris Basham.
“He’s not okay,” said Van Gaal. “I think he shall be injured but we have to wait and see.
“It is more of a knee problem than an ankle problem. Everyone thinks that it is his ankle that was blocking the shot of an opponent but I think it is more of a knee problem.
“But he is playing better and better.”
De Gea 6, Darmian 6, Smalling 7, Blind 6, Borthwick-Jackson 6, Schweinsteiger 6, Herrera 6, (Lingard 60, 6) Fellaini 5 (Pereira 77, 5), Mata 6 (Depay 60, 7), Rooney 6, Martial 5.
Romero, Varela, McNair, Keane.
Long 7, Brayford 7, Edgar 7, Collins 7, McEveley 6 (Reed, 77, 5), Coutts 7, Basham 7, Hammond 6, Woolford 7, Sammon 6 (Done 90), Sharp 6 (Flynn 64, 6).
Howard, Baxter, Campbell-Ryce, Adams.