Liverpool thrashed Spartak 7-0 in the final group game in the Champions League.
"Their defence was never really tested but I still think they are potential winners of this tournament," Duff told RTE.
"They get found out over a league season over 38 games, but now they are in the knockout stages. Over two legs, they can beat anyone. We've seen what they can do in 20 minutes. They just steamroll teams with their energy, with the fab four as we call them.
"Over two legs, Real Madrid, Barca, I think they could do anyone. I honestly do."