The U's, who saw off League Two side Bury in a first-round replay on Tuesday, are riding high at the top of the Skrill Premier having lost just one of their opening 20 league matches this season.
And Blades midfielder Doyle knows he and his team-mates will have to be at the top of their game to avoid a giantkilling in a televised away clash.
"Cambridge are obviously dangerous. They're leading their division and I know a fair bit about Richard Money (Cambridge manager)," Doyle told United's official website.
"He gets his teams organised and, more importantly, winning games on a consistent basis.
"Any team in any division that has only lost once by early December after 20 league games deserve respect and we won't underestimate the threat they will pose."
Doyle added: "These are the games where, if you're not completely at it, then you are going to get found out.
"You've got to relish these situations and be right 'on it' because character could well be the key."