Wayne Rooney
Rooney's late penalty saw United sneak into the fourth round of the FA Cup.Getty

Wayne Rooney was "lucky" to score Manchester United's decisive penalty in their FA Cup win against Sheffield United – as Louis van Gaal had already substituted his first and second choices from the spot.

United did not register a shot on target until the 69th minute in front of a frustrated Old Trafford crowd in their third round clash against League One opposition. Memphis Depay, who came off the bench to replace Juan Mata after an hour, broke into the Sheffield United area in injury time and was clipped by Dean Hammond, allowing Rooney to convert the crucial spot kick.

But had Mata and Ander Herrera, who was also replaced on the hour by Jesse Lingard, remained on the pitch, the United skipper wouldn't have had the chance to win his side the game.

"He [Rooney] was the third the man, I changed number one and number two (Mata and Herrera) so he was lucky," van Gaal told BT Sport post-match.

United registered just two shots on target all game with a strike from Depay after 66 minutes prompting ironic cheers from home supporters in attendance.

Van Gaal has previously indicated he will try to remedy the club's shortcomings in the final third during the January transfer window but appears to have made a U-turn in that approach, suggesting he is no longer keen to add to his squad.

"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."