Wayne Rooney scored a penalty in the third of six added minutes to spare Van Gaal's blushes after Memphis Depay went down in the box following a challenge by Dean Hammond
The 1-0 victory and the manner in which it was earned will do little to silence the Dutchman's doubters but he defended United's performance
"It's always difficult to play against such an opponent with 10 or 11 players behind the ball," Van Gaal told BT Sport 2
"You have to play more quickly than we did in the first half
In the second half it was a little bit better but not too much because they were very well organised.
"It's more easy to defend than attack and at the end we get the penalty but it was a deserved penalty.
"We win this match and that's the most important thing
I said in advance to the boys, 'The only thing that counts is we are in the second (fourth) round', and we did it.
"How many chances have Sheffield United created? Nobody is talking about that
Everybody is talking about the fantastic defence of Sheffield United
I don't think you can do different as a player of the second (third) level."
Meanwhile, Van Gaal suggested he would not add to his squad during the transfer window.
"I don't think it's very wise to purchase players in January," he added.
"When it is possible and we think he can give us a contribution of our level then of course we shall do it.
"But normally we don't do that
We didn't also last year, I believe."
Source : PA