De Gea watched from the stands as United kicked off the Barclays Premier League season with a narrow, yet comfortable, win over Spurs thanks to Kyle Walker's 22nd-minute own goal.
Sergio Romero, one of five United debutants, played the whole 90 minutes in goal and 22-year-old Sam Johnstone sat on the bench because Van Gaal felt De Gea had been distracted by talk of a move to Real Madrid.
The Dutchman was reasonably satisfied with Romero's performance, but would prefer to have De Gea in goal for United again soon and still hopes the Spaniard will stay.
"Of course," the United manager said when asked if De Gea could play for the club again.
"We have to pay a lot of money for the players we want so we want to sell at the same level when we have to sell.
"We want to keep him, of course
He has been voted the best player here for the last two years."
Romero, a free signing who joined the club less than two weeks ago, had little to do for much of the match.
The Argentinian pulled off a crucial block to deny Walker before United took the lead and also saved two good efforts from Christian Eriksen at the death as Spurs looked for an equaliser.
Van Gaal was impressed by the 28-year-old's performance given the short amount of time he has had with the club.
"We have observed (De Gea) in preparation and that was not the same David de Gea as last year," Van Gaal said.
"Of course when the situation has been cleared after September 1 we will look again
But I'm happy Sergio did well
He saved us twice.
"We have to improve his building up, but Sergio trained only two days with his fellow players."
Romero's fellow new signings Memphis Depay, Morgan Schneiderlin and Matteo Darmian also all started the game and Bastian Schweinsteiger came off the bench to make his debut too.
Of the new faces, Van Gaal was impressed by right-back Darmian the most.
"I said in advance to the boys you don't have to be very eager to play this match and with a lot of passion - you need to use your brain also," said Van Gaal, who turned 64 on Saturday.
"When I have to pick up one player I think Matteo was the best because he played in ball possession
He doesn't lose the ball."
While Van Gaal headed out to celebrate his birthday, Mauricio Pochettino travelled back to London a disappointed man.
Once Eriksen had squandered a good early chance, Spurs never looked like scoring and, unfortunately for the Londoners, they went behind when Walker poked the ball past Michel Vorm in the first half just as Wayne Rooney was about to shoot.
"Today we were unlucky
It was very frustrating and disappointing in the dressing room," Pochettino said.
"In the first 20 minutes we dominated the game and created a chance.
"After that mistake we conceded a goal.
"I think we created more chances and deserved more than to lose 1-0.
"I think we played well and a high level
But it's true that when you make a mistake when you have the player in front of you like Rooney, like Depay, like (Ashley) Young then will get a chance and score."
Harry Kane failed to test Romero all afternoon and Pochettino admits he needs more firepower up front.
"Yes, this is very clear," he said
"We need to add more players."
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Source: PA