"Hayes wants to win the Champions League. She knows to do that you must have internationals at every level," former England goalkeeper Rachel Brown-Finnis told BBC Sport. "Lyon have that, Barcelona have that."

Berger, shortlisted for the Best Fifa Women's Goalkeeper Award, has produced some stellar performances this season - the standout coming against Lyon in the Champions League quarter-finals, where her penalty shootout saves earned Chelsea victory.

This has come against the backdrop of her having treatment for thyroid cancer for a second time, after the illness returned during Germany's run to the Euro 2022 final, making nights like the one against Lyon even more remarkable.

"She has done phenomenally well to come back," Siobhan Chamberlain, another former England goalkeeper, told BBC Sport. "I cannot imagine what she has been through.

"You need back-up if she needs time out. If you have one missing, you need two top-quality keepers.

"You can have a kid on the bench, but if you really want to win titles all the squad needs to be capable at the top level."

How do you cope with being number two?

Zecira Musovic has started in goal for nine of Chelsea's past 16 fixtures

Yet Berger, 32, has been susceptible to errors this season, notably in the 1-1 Champions League draw against Real Madrid in December, when her clearance fell to the feet of Caroline Weir who accepted the gift and slotted into an empty net from 22 yards.

Three days later, Berger conceded two goals in as many minutes against Reading, allowing Sanne Troelsgaard's shot to squirm under her body for the first.

This saw Musovic, 26, come in for the next six games, including the 1-1 league draw at Arsenal when the Swede made a series of fine saves before Sam Kerr's late equaliser.

Although Berger returned to the team in February, Hayes has continued to swap her keepers, with Musovic playing against Brighton and Aston Villa in the Women's Super League (WSL), as well as the FA Cup quarter-final victory over Reading.

While in other positions substitutions and rotations are more common, not so among the goalkeepers. The balance between supporting team-mates and the desire for first-team football is more delicate than anywhere else on the team - something Brown-Finnis has first-hand experience of.

"I was England number one, but I was injured at Everton and the other keeper did a good job so the manager kept her in," she said. "Communication is crucial. You have to understand the other keeper is in for a reason.

"If the manager is communicating that while you're out injured, you accept that decision. The clarity of communication has to be the biggest thing, to understand where you are.

"With England, I've been number one, two and three. In the grand scheme of a career, you have to accept that - not put your feet up, but work out what you have to do to work harder."

Musovic addressed this in an interview with the Chelsea matchday programme in February, saying: "We are all extremely competitive but we also know that we are a team within the team, and we need to support each other.

"I'm just trying to be the best version of myself, and that includes being a good team-mate and supporting them. I know that when they do it well, I need to do it even better, so we drive each other to the best state possible."

What neither keeper can afford, says Chamberlain, is to allow their personal ambitions to hamper the team, or to allow a desire for regular game time to descend into ill will towards their counterparts.

"You don't want the keeper ahead of you to make a mistake, as you don't want that as an individual yourself," she said. "It's about wanting to drive yourself to be better than them - it might be five games, or five months, but you need to drive the standards up rather than drag someone else down.

"If you have that, then the goalkeeper union will thrive."

And Evrard makes three

Chelsea have agreed a deal to sign Belgium goalkeeper Nicky Evrard from Leuven

If it was tricky enough to choose between two international keepers this season for Hayes, next season she looks set to have three.

Berger and Musovic, who have contracts lasting until 2024 and 2025 respectively, will be joined by Belgium's Nicky Evrard, 26, who has signed a pre-contract agreement with Chelsea.

"Evrard would not have signed unless there was clarity and distinction between keepers, trying to win silverware as a starter or otherwise," said Brown-Finnis.

"I get why you would question why a player would want to arrive not as number one, but that is the life of a goalkeeper."

Chamberlain says Hayes' reputation for player management will be crucial - but keeping all three happy could be difficult.

"When you have three international keepers, their countries will be saying you need to be playing," she said.