The Denmark forward has struggled to find his feet at United over the past 12 months.
With Erik ten Hag sacked and Ruben Amorim soon arriving, Van Nistelrooy believes Hojlund can go on to be a key United player.
He stated: "My advice? Stay calm and believe in himself, believes in his qualities.
“Like I do and his team-mates do. He knows he's young and he's promising and the daily work he puts in will pay off in the end. Not only for him but his team-mates. And I'm confident in that. I mean, most goals are scored in the penalty area, it's where we want to get our players as much as we can, but it depends on the games.
“There have been games where we've arrived many, many times, creating many chances. Unfortunately, we couldn't convert them all and that cost us a couple of results. But you have to deal with circumstances. When it's needed, to hold up play and get players into the third-man position and play on from there, he does a brilliant job (with that) as well.
“And, of course, he is a guy who can change a game in a second, like with the penalty moment (against Chelsea). We just need one or two in the box, and you can be him but it can be the others as well, so that's a threat we always have."