Watford had the better of the fourth round encounter at Vicarage Road but Rafa's 30-yard strike towards the end of the first half saw us home by the narrowest of 1-0 margins.
The Hornets had chances to equalise but found Carlo Cudicini in fine form.
The goalkeeper made two outstanding saves, tipping Sean Murray's curler onto the post and then denying Nyron Nosworthy from point-blank range.
Watford hit the post again and both Kyle Walker and Michael Dawson made goal-saving blocks as we crept through.
"It was a bad game from us," said Rafa. "It was a difficult night but we all know the main thing about the FA Cup is to get through and that's what we did.
"Watford played well, we didn't keep the ball as we usually do and then you can see why it was difficult.
"These cup games are always hard but we won and that's the most important thing."