Mary Fowler ruled out of Matildas' World Cup clash with Nigeria

Defender Aivi Luik, who was not used in Australia's gritty 1-0 win over Republic of Ireland in their tournament-opening match at Stadium Australia last Thursday night, has also been ruled out of the Matildas' second outing.

Fowler and Luik sustained mild concussions in separate incidents at Matildas' training on Tuesday in Brisbane.

Twenty-year-old Fowler started in place of injured captain Sam Kerr (calf) against Ireland and looked dangerous on the ball when she got it.

Football Australia said in a statement that both players had "fully recovered" and commenced Football Australia's Graduated Return to Play protocol.

Forward Kyah Simon remains sidelined with a knee injury but Tameka Yallop told media this week she was "good to go" after missing the Republic of Ireland match with a thigh complaint.

When asked if he regretted Simon's selection, coach Tony Gustavsson told Optus Sport "definitely not".

Football Australia said Fowler and Luik, who wasn't used against the Irish, were fully recovered and in return-to-play protocols.

"We played an intense tempo game in training and unfortunately we had a couple of head knocks," Gustavsson said.

"It's up to the medical team to decide whether players are available or not."

Football Australia's concussion protocol rules players out for at least six days after a head knock, which opens the door for Fowler and Luik to play in the final group game against Canada in Melbourne on July 31, provided they clear the return-to-play program.

Fowler was walking laps at training on Wednesday and started running later in the 15-minute window available to media.

A week on from injuring her left calf, Kerr was walking laps on Wednesday, the most activity she has carried out in public since the injury was announced.

Kerr will be reassessed after the Nigeria game but given the tight, four-day turnaround to the Canada clash she appears unlikely to play.

The striker and her teammates have insisted Kerr will return to action during the tournament.

Simon was also walking laps at Brisbane's Perry Park on Wednesday but Luik was not.

With Gustavsson unlikely to change the successful midfield pairing of Kyra Cooney-Cross and Katrina Gorry, experienced playmaker Emily van Egmond could line up alongside Foord.

Tameka Yallop, who has recovered from a corked thigh, and Alex Chidiac are other options to come in, while Cortnee Vine and Hayley Raso are the first-choice wingers.

Defender Alanna Kennedy was back in main training after missing Sunday and Monday for loading reasons.

A win against Nigeria, who drew 0-0 with Canada in their first group match, will seal a spot in the knock-out stages for the Matildas.

The Matildas round out the group phase against Canada, who play Republic of Ireland in Perth on Wednesday night, in Melbourne on Monday (8pm).

- with AAP