The former Hammers defender believes the 22-year-old England star would be "the ideal fit" for United but with the youngster expected to be a target for a number of big clubs over the summer, he thinks he'll consider other options first.
Glen told BettingOdds.com: "I don’t think Manchester United could get anyone better than Declan Rice. If he was available, I think Chelsea or Liverpool would be in front of them anyway.
"Those players only come around once. United have got [Donny] van de Beek or whoever and they try to sign all these players in that position for loads of money and it’s just never worked."
Glen doesn't believe Manchester United have the same appeal for players as they used to.
He added: "If I was Declan I would be a bit concerned to be honest with you. I’d be thinking Manchester United aren’t the Manchester United of old.
"There used to be a time when, anytime Manchester United came calling, you go. It was the number one place. Everyone wanted to go. But they don’t have that appeal anymore.
"I don’t think he’d end up there whether they want him or not. But in terms of their business, I think they should just sit tight and wait until the summer and weigh the whole situation up with everybody."