The 20-year-old has struggled in his first two games of the season, making errors against both Manchester City in the Community Shield and West Bromwich Albion last weekend.
The Welshman has now admitted that Tottenham are preparing to add to the goalkeeper's "bad time".
"It's obviously going to be nerve-racking for [de Gea] to go to United and be in the first team, but he showed at Atletico Madrid and with Spain that he's a world-class keeper," Bale told the The Mirror.
"If any keeper's having a bad time, you're going to test them and it's going to be no different.
"We're going to go to Old Trafford and give our all and test their whole team."
Bale joined Tottenham from Southampton in 2007 and has made 77 league appearances for the club.