The Republic of Ireland international explained how Martinez worked over-time to convince his players they could win for the first time in 21 years at the home of Manchester United.
"It's been too long really and the manager has been working hard to get us to believe we could come here and get a result," he told the Liverpool Echo.
"It came on the way we've been playing, the way we've been keeping the ball and the confidence that has built. He just wanted us to play with no fear.
"Earlier in the season we tried to go to City in the same way but since then the manager has had more time to instill belief in us and we're delighted with the result.
"From the sidelines all we could hear was the manager telling us to keep getting on the ball, and that was when we were 1-0 up.
"We got a brilliant result against Stoke and worked hard in training on how to beat United. Thankfully we did."
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