Marney said: "Look at the quality of Geo and the goals he has scored for us, he's been unbelievable.
"He has a great record against United, too. We know it will be tough there but, with someone like Geo in the team, it gives you confidence because he is such a great finisher.
"He was a great signing for us by the gaffer who picked him up on a free. And hopefully he can carry on with his current form."
Hull face United just three days after their 3-0 hammering by Chelsea.
But Marney insisted: "Everybody was thinking playing Arsenal and Tottenham away would be tough. But we're getting used to playing world-class players every week."