Despite suggesting that the club have missed out on being able to sign Paul Pogba or Toni Kroos, Butt believes that Herrera, a £28.4m signing from Atletico Bilbao, can be a success after scouting the midfielder as part of his role with United last season.
"He was at the level where you just had to take note of this player because there are not many top midfielders you can get," Butt told the Manchester Evening News. "You can all talk about Pogba or Kroos but these kind of players are not easy to get. So you have to widen your net and Herrera is a bonus.
"He is at a level where he can play at a club like United and given a bit of time he can excel.
"I watched Ander a few times last season on scouting trips. He impressed me. He can become a star player for us. He can become a big influence on our midfield, [but] you will have to be patient with him."
Butt will reportedly be informed this week whether he will be needed by Louis van Gaal this season.