De Gea will once again be in the stands when United take on Newcastle at Old Trafford on Saturday because Van Gaal feels the Spaniard is still not able to focus properly because of Real Madrid's interest in him.
According to reports in Spain, the 10-time European champions will make one last attempt to sign the goalkeeper before the window shuts on September 1.
But after a week in which United's reputation took a blow following Pedro's move to Chelsea, Van Gaal came out fighting at his pre-match press conference on Friday.
The Dutchman insisted the lure of United remains as strong as their desire to hold on to their two-time player of the year.
"I think when Manchester United wants a player he shall come, besides when a club is not selling," the United manager said at a spiky press conference.
"It is the same with us
We are not selling De Gea."
Van Gaal launched a passionate defence of executive vice-chairman Ed Woodward despite the fact that former target Pedro signed for Chelsea this week, rather than United.
''I have the idea you want to separate Ed Woodward with me
Never
Never,'' Van Gaal said.
''Because every day I am updated by him and I trust him.
''I don't think they (the critics) have to doubt Ed Woodward because he has proven already for many, many years he is the right person on the right spot.''
If history is anything to go by, Van Gaal will be able to rely on one of his current crop to find the goals against Newcastle on Saturday.
Wayne Rooney has scored 11 goals in 17 Barclays Premier League appearances for United against the Magpies.
The United skipper, who is yet to find the net this year, admitted in midweek that his performance in the 1-0 win over Aston Villa was not good enough.
Van Gaal was encouraged to see honesty from his skipper, and backed him to hit form soon.
"When you are saying so publicly that you are so bad then you know you are very bad at that time," Van Gaal said.
"I think it's fantastic that a player admits that
For me it's unbelievable because he is one of a few players saying such things.
"He is taking a risk, but he is very confident that he shall make the goals we expect from him and I am confident also."
Source : PA
Source: PA