Megan Rapinoe may not be a first-choice starter for holders the United States at the Women's World Cup but new co-captain Lindsey Horan is banking on the veteran superstar to play a key role nevertheless.

The 38-year-old Rapinoe, a central part of the USA sides that won the 2015 and 2019 World Cups, is at the tournament for the fourth and final time before retiring.

With a wave of fresh attacking talent breaking into coach Vlatko Andonovski's squad, Rapinoe might not play as often as before.

But she remains an inspirational presence on and off the pitch for a side targeting a historic third consecutive title.

"She was always a role model for me growing up," Horan, who is sharing the captaincy with striker Alex Morgan, said ahead of Saturday's World Cup opener against Vietnam in Auckland.

"She is just a different kind of player, she is so intelligent, the way she moves the ball and thinks about the game.

"To send her off on a high would be incredible and then it goes without saying everything she has done off the field as well," Horan added of the former Ballon d'Or winner.

Rapinoe was a key voice in the team's battle with the US federation for equal pay and conditions and remains an unflinching advocate for women and the LGBTQ community.

"She has been a huge voice for this team, for women, for everyone in our country, everything that she stands for. She is one of a kind," Horan added.

"She may have a different role from the last World Cup but everything she does is so important."

Andonovski said Rapinoe's new role "is not any smaller" but added she would likely not be fit to play all of the game against Vietnam.

Rapinoe, who has 199 caps, has been recovering from a knock picked up playing for her club OL Reign last month.

"She is very important for us, she understands her role and we know that in order to win this World Cup we are going to need the best of Megan Rapinoe on the field and off the field," Andonovski said.

He added: "Her minutes are probably going to grow as the tournament goes by."

The USA squad features 14 players appearing at their first World Cup, with Sophia Smith and Trinity Rodman two young attacking stars to watch.

Meanwhile Horan, of French champions Lyon, has benefited from an injury to veteran centre-back Becky Sauerbrunn to become co-captain.

"I spoke with Becky a little bit, who gave the best advice -- to embrace this role," Horan said.

"It is a huge responsibility but such a great honour."

The USA hammered Thailand 13-0 in their opening World Cup game four years ago and are expected to win big against Vietnam.

Harder challenges await later on in the shape of an ever-improving batch of European sides in particular.

"I always say the US has held this standard for ourselves. We have been on top for so long and stayed there consistently on top of the world," Horan said.

"Now teams are getting better, there is more investment. Teams are getting closer to the US, but this is what is supposed to happen."