The midfielder was set to be a substitute for the Premier League clash, which the Reds won 2-0 at Anfield, until Naby Keita sustained a groin injury during the warm-up.
That setback forced a change to the starting XI, with Milner brought in just minutes before kick-off against the team placed eighth in the table.
"I didn't find out until we went into the dressing room after the warm-up," the vice-captain told Liverpoolfc.com after goals from Mohamed Salah and Sadio Mane earned a 19th win in 20 league games this term.
“But the boys are always prepared, we're ready to go. We always do the right things so it's not a problem for anyone to step in.
“If you're mentally switched on anyway, I don't think it's too much of a big deal. If you just feel you're a sub, you're not ready – but I don't think anyone has got that mentality here. I'm pretty sure all of the lads would be able to do it."
Milner added: “We knew it would be a tough game, they're a good team. It's no fluke where they are in the league, they are prepared and have high standards. They pick up points everywhere they go, so we knew it would be tough.
“It was a good team performance, some good goals in there and we could have had a couple more."