Having just returned to the court after her own serious knee injury, Cullen said she was devastated to see Kopua suffer a similar fate.
"Obviously with having had my own issues with my knees I just felt sick when I saw it on TV," said Cullen.
"I can't handle watching anyone go down with injuries, let alone knee injuries -- and to see it happen to Casey was horrible. But she is tough and she will bounce back for sure."
After several setbacks with her recovery, Cullen returned to fully contested training only a week out from last month's national trials. Still getting her confidence back in her knee, Cullen rated her fitness at the time at about 70 per cent of where she was before she ruptured her ACL on the eve of last year's World Youth Cup in Glasgow.
But having given the joint a thorough workout at the Lion Foundation National Champs this month, Cullen now feels much readier for top-level netball.
New Zealand coach Waimarama Taumaunu didn't offer the youngster an opportunity to second guess herself anyway.
"She just said, 'Kayla, we need you to join us in camp at 5.30pm today,' so there wasn't room for hesitation," said Cullen.
The 22-year-old rejoins the team at a tough time. Not only are the Ferns dealing with the trauma of losing their captain, they are also struggling on the court, suffering three big losses to the Diamonds over this series to stretch their losing run against their transtasman rivals to eight games.
The Ferns are under pressure to redeem themselves with at least a more competitive showing in tomorrow night's final clash at Trusts Stadium.
"It's definitely a challenging time for the team," said Cullen.
"I'm not trying to fill anyone's shoes or be the hero or anything. If I do get the opportunity I'm just going to do my best to make the most of it and bring my own kind of flair or freshness to the game."
Cullen last took the court against Australia in 2012 in her first full season in the Silver Ferns environment.
"I remember it just being so intense, and they seem to have only got fitter, faster and stronger since then," she said, "so it is going to be a massive step up from the netball I've been playing lately."