The last spot-kick Rooney took was successfully converted to make him England's all-time leading scorer, but his struggles at club level continued on Tuesday night as Boro stopper Tomas Mejias saved from him and Ashley Young while international colleague Michael Carrick also fired over
Only teenager Andreas Pereira scored from 12 yards following a goalless 120 minutes, yet Van Gaal was not prepared to attribute responsibility to his three England internationals that failed to find the net.
"In the goal was a goalkeeper; and he was very quick and he gambled also," the Dutchman said.
"He's 195 centimetres
That is not easy to do when the goalkeeper is gambling for the right corner.
"You need players who look at the goalkeeper and put it in the other (corner)...very early he goes away
It is already 120 minutes you have played so you are not so sharp any more
These things happen."
Of more concern to the United boss was his men's wastefulness during the 120 minutes that preceded the penalties.
Rooney, Young and Anthony Martial were all called for from the bench inside 71 minutes as the hosts failed to break down their Sky Bet Championship opponents in normal time.
Marouane Fellaini then twice missed the target with close-range headers, a feat Martial was also guilty of, as Boro held on to force a shoot-out where they would come out on top in front of 10,000 away supporters.
"When you are not finishing the chances - and they were very easy chances in front of the goal or in front of the goalkeeper - then it is very difficult to win a game because the goals are counting, not the chances," added Van Gaal.
"You can say it is unlucky but that I have said already after the match against (Manchester) City also because that was also 0-0
You have to force the luck also, we didn't do that because we didn't score
That's why I'm disappointed."
Boro progressed to book a last-eight home tie with Everton and boss Aitor Karanka assured his players they outperformed the 20-time English champions on their own patch.
"I told my players after the game that we were better than them - we were more confident on the pitch, we were fitter than them," he said.
"For that reason, I have to say how proud I am of my players
Penalties sometimes you lose - last season we lost against Liverpool after 30 penalties - and sometimes you can win.
"The main thing is the performance, and the performance against Man United, a big team, it was amazing."
Source : PA
Source: PA