Greenwood, 17, made his full Premier League debut in defeat to Cardiff City on Sunday.
“Mason is a phenomenal talent,” Butt told Inside United. “He’s done unbelievably well for me in the Under-19s this year and done well when he’s gone up into the Under-23s, he’s playing for England and he’s had his first-team debut.
“They have to be ready and it’s not where they are now but where they are in 18 months.As a club, we have to make sure we don’t go overboard on players like Mason. Yes, they’re very talented. Yes, they’re unbelievably good players. Yes, we hope they play for Manchester United. But how many times have we said that about a player in the last 15 years?
“We are very confident that Mason and the rest of the lads we talk about will have good careers here. But there has always got to be a reality check. It’s got to be a case of okay, as and when it happens, it happens. In the meantime, all the process is to get to where he wants to be when he’s 20 or 21, not where he wants to be at 17 or 18.”