Leah Williamson has been ruled out of the second leg of Arsenal Women's Champions League second-round tie against Hacken due to concussion protocol.
Williamson missed the Gunners' WSL opener against Manchester City last weekend, which ended in a 2-2 draw at the Emirates Stadium.
Lotte Wubben-Moy, Williamson's England team-mate, and Laia Codina, who won the World Cup with Spain last year, are in line to continue in defence as Arsenal look to overturn a 1-0 deficit from the first leg.
Victoria Pelova and Steph Catley were also absent from training on Wednesday, with the latter also ruled out of Thursday evening's clash.
Manager Jonas Eidevall said when asked about Williamson's absence: "I don't have a timeframe on Leah's injury at the moment. It's guided by our medical team.
"When I have something more concrete to communicate, I will do that. She sustained the concussion during an incident in training.
"If we don't qualify, I would definitely consider that a failure. We need to get into the groups."
Eidevall confirmed Lina Hurtig is still missing through an unspecified injury but is back in training.
The Gunners boss added: "Steph Catley also missed training, so she's also confirmed she can't take part. And Lina Hurtig, she was in training. She's making progress, but she's not far enough ahead to be able to make the squad tomorrow either."
'Positives to take from Man City draw'
Substitute Beth Mead scored a late equaliser to rescue a 2-2 draw for Arsenal against City in a thrilling opening-weekend Women's Super League clash at the Emirates Stadium.
And Eidevall saw plenty of positives from his side as his side prepare to face Hacken.
He said: "I think that was important because I didn't think our performance and result against Hacken was a step in the right direction for us, and that happens in football.
"Everything is about how you react, how you move forward as a team. I think our reaction to that game was really strong.
"I thought we performed well against Manchester City. I think we were the better team for the full 90 minutes in that game. And I don't think there are many teams that can be that against Manchester City.
"And we need to draw confidence from that performance, but then we need to understand it's a totally different game here now on Thursday. It's knockout football. It's against a different team.
"You get nothing for free in football. You have to work for everything. You have to create everything again, and we need to be prepared to do that work Thursday night."