The 18-year-old fullback joined the Matildas just two days before the World Cup kicks off in France to replace Laura Alleway, who has suffered a suspected foot fracture.

Roestbakken recently led the Young Matildas through an undefeated campaign in Myanmar to secure their place in this year's under-19 AFC Women's Championship.

Young Matildas coach Gary Van Egmond speculated the uncapped teen would make her international debut at the World Cup following standout performances in Asia and the W-League.

The Canberra United fullback already has three W-League seasons under her belt and earned the December nomination for the Young Player of the Year award.

Canberra United coach Garriock said the promotion was well-deserved after a breakthrough domestic season.

"Karly was consistently one of our best players this past season so to see her get this call-up is really great news," Garriock said.

"The injury to Laura [Alleway] is a big shame and I wish her all the best, but I look forward to seeing Karly take this incredible chance with both hands."

Alleway's untimely injury means she will miss out on playing at her third World Cup after suffering a suspected foot fracture.

The 29-year-old had scans in Turkey and the Netherlands on her injured foot but won't be ready for when the Matildas kick off their World Cup campaign against Italy on Sunday.

"This is an unfortunate situation for Laura Alleway and as a group we feel for her," Matildas head coach Ante Milicic said.

"We have given her every opportunity to return to full fitness ahead of the tournament, but with our first match now just days away we needed to make a call which has provided Karly Roestbakken a fantastic opportunity."

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Canberra United teammate Laura Hughes was one of the first to hear about Roestbakken's promotion to Ante Milicic's squad.

Roestbakken sent her a Snapchat with the caption "I'm going to Holland" moments after she received Milicic's call.

"I didn't realise that Holland was the Netherlands but then it clicked," Hughes admitted.

"We FaceTimed and went mental. I'm so proud of her and she's worked so hard for it.

"She's excited to bring something new to the team and hopefully they win the World Cup."

Former Matilda and Canberra United player Grace Gill believes Roestbakken is "well primed" to earn her first international cap on Sunday.

"She would have been a prime prospect for filling out the Matildas squad," Gill said.

"It would be huge if she got minutes on the park. She's really fit and has got a great engine in her.

"To have the opportunity to step up and head over to France is huge. It's fantastic for Canberra and for Karly, I'm really proud of her."

The Matildas kick off their World Cup campaign against Italy at the Stade du Hainaut on Sunday, 9pm.