Ward was booed by the home support after errors for Wolves' first and third goals.
Van Nistelrooy said: “It doesn’t make it easier for him and for the team. We all understand disappointment from the crowd but it shouldn’t go to one player.
“Booing and being disappointed with a result, I think it’s a right of a supporter to show your emotions. We all do that. But when it goes to one person, that is hard to see for me personally.
“I think he kept his composure very well over the course of the game. It’s not easy when something happens like that, when you catch a ball and people start cheering, and boo you, when it gets personal like that. I think he handled it very well.
“He’s the first one to look in the mirror. They all want to do well and perform and do their best for the club. When it doesn’t work out, it’s good to stick together as a squad and as a club.
Asked if he considered a substitution, the Dutchman also said: “No. I’m sticking with players in these moments. You have to address certain situations in your team selection, but no it didn’t cross my mind. He handled himself really well. It must have been hard. We all felt that. Hopefully he can turn things around quickly.”