Romero completed his move to Man United on Monday by signing a three-year deal.
Meanwhile, De Gea has been strongly linked with a move to Real Madrid for a number of months.
Van Gaal admitted the club are preparing themselves for De Gea’s potential exit by bringing in Romero.
When asked why he had signed the 28-year-old, van Gaal said: “I know him very well and I think in our situation, if De Gea is going or not going, it is always a threat and we have to take care, make sure that we are prepared and that is why I think it is very good that Sergio Romero is now a Manchester United player.”
Romero, who is expected to replace Victor Valdes rather than De Gea after he fell out with van Gaal, will be United’s second-choice keeper for the foreseeable future.
If De Gea ends up leaving as well, Ajax’s Jasper Cillesen is said to be a target. The signing of Romero should trigger a keeper carousel at Old Trafford but fans will be hoping it doesn’t involved De Gea.