The pair have struggled so far after their summer arrivals.
But Van Nistelrooy says: "I'm convinced about Manuel that he will be a great player for United. He had a very good performance yesterday and against Fenerbahce he had a good performance. I think, of course, getting players in an early conclusion, players who are here for two or three months, we should be careful with that I think.
"It's a great signing, he's a very good player, professional, he has very specific qualities in midfield when he gets in between players and recover the ball for us to get us in attacking positions. Also on the ball, very decent, so I mean he will be a very good player for this club.
"The recruitment is not part of the assistant manager, but I can comment on my work with Manuel on a daily basis. He's adjusting, he wants to adjust and be part of this squad.
"He wants to do well for this club and is very motivated to do so. He is getting better every time and the performance yesterday was good and we can build on that. We are very thin in midfield and we're going to need him in the coming games, which I think he'll do well in."
On Zirkzee, he continued: "Josh is a similar answer to Manuel Ugarte's situation. Josh came in and he's a young player with a lot of potential.
"He is a typical No.9 in the sense that his hold up play is very good, he's very strong in combination play. He's a different type than Rasmus because Rasmus is more of a No.9 who plays high up the pitch and he's getting in the back of defenders.
"Josh is more of a player who is coming toward the ball and gets the team playing. In the Crystal Palace game, I thought he was excellent and showed one of his best performances.
"Another one with a lot of talent, he's a good age and has a lot of potential to develop. We have to look after him, give him time because he will get better and will be a very good player."