The Brazil international, 26, could head back to Camp Nou from Paris Saint-Germain, which has reportedly agreed to let Neymar return to Spain for €200million.
Neymar scored 105 goals in 186 games for Barca before leaving for PSG, where he is in his second season.
Former Barcelona manager Van Gaal feels re-signing Neymar would be a bad move for the LaLiga club.
"I think it's not good for Barcelona, but that's my opinion," the Dutchman said. "Neymar is still a young player who can continue to develop. He must first learn to be a team player.
"Neymar is too individualistic, which is why I think Barcelona should be vigilant before deciding to get him back."