The Spaniard's future at Old Trafford remains uncertain with the Spaniard continually linked with a move to Real Madrid and Van Gaal has already suggested that the goalkepeer wants to leave Old Trafford.
De Gea has just one season left on his United contract, but the club have so far resisted overtures from Real to buy the 24-year-old despite being resigned to losing their first-choice goalkeeper.
"We decided together - David and the staff - to take the decision not to play because a goalkeeper is more concentration and focus," Van Gaal told BT Sport.
"He maybe gets only four or five times that he has to save an attack, and of course a lot of building up play, but it's more focus and concentration.
With Van Gaal publicly questioning Victor Valdes's commitment at United earlier in the summer, the manager has opted to hand summer arrival Sergio Romero his debut with youngster Sam Johnstone on the bench.
Van Gaal continued: "In spite of the fact that David de Gea wants to play, we think that it is better for him, but also for us because with Romero it's a short time [since he joined].
"So for me it's not normal that he is playing but I think in circumstances as this, [it is] the best decision - and it is a very difficult decision because I think Sam Johnstone did very well [during pre-season].
"But I think with his experience, [Romero] having played already at this level, this is the best decision."
