Nunez replaced Roberto Firmino with half an hour left in the game and was involved in the incident that led to the decisive penalty when the game was tied at 1-1.
He then capped a dynamic display with a goal to clinch the trophy for Liverpool as they inflated another defeat on City.
“He’s a top player, a young player who is willing to learn,” said Alexander-Arnold of Nunez. He’s bonded well with the lads. He came on with a point to prove.
“Who knows what will happen over the course of the season? Our aim is to be as fit as possible to push them from the very first kick. This was the perfect start and we hope to win trophies along the way.
“He won the penalty, scored a goal and looked very lively. He’s been bought in to score goals and he’s proved he can do that in this game.”
Liverpool manager Jurgen Klopp suggested both his side and Liverpool will improve when the Premier League season gets underway, as he joined the chorus of praise for his latest signing.
“Nunez is good. But today we played the smarter passes for him and that was crucial,” declared the Liverpool boss.
“We said he needs balls from the sides after the first two games [of pre-season]. Today we saw he was involved in every situation of the second half.
“He opened things up for us, we could use the spaces, and he was there in the finishing moments. He had a good game.
“Overall, teams who were far away from their best, but what both teams made of what was had was absolutely incredible.
“I liked the game a lot, most of the time City games against us are good to watch. We don’t win them always, but they are always entertaining.
“I was happy with the starting line-up and how we started the game, but then I have to say that all subs made a real difference.
“They were really in the game, and I loved that. The thing I learned today was that the boys are ready. That was good.”
City manager Pep Guardiola and his players refused to accept their runners-up medals after that game, as he insisted he was happy with his side’s performance.
"Congratulations to Liverpool. The first 15-20 minutes they were better. After we got our game and it was quite similar the last 25 minutes, we arrive when they left space and tried to attack,” he said.
"I said to the players, I am proud of how they gave everything.
"We have to improve. The timing is not much what we have to do, especially the fact we need this month training and games to get our tempo and we will do it."
“It is good for him to see the reality in a new country and a new league but he was there for chances.
“We still have a week to go and we will try to be ready for next weekend.”
Guardiola also defended striker Erling Haaland after a horrible late miss for his new striker as he added: "He had chances - two or three in the first half and one at the end. He fought a lot and made the movements.
"It’s good for him to see the reality of the new country, new league. He was there. Today he didn't score - another day he will score."