In a massive boost to Australia's hopes and morale, Kerr on Saturday said she available for selection at AAMI Park on Monday night after missing the opening two games through a left calf injury.
But just what her role will be remains a mystery.
Australia, on three points, must beat Canada, who have four to guarantee passage to the round of 16.
A win could even deliver top spot in Group.B.
Otherwise, they have to draw and rely on Ireland, still pointless, to beat Nigeria (four points) in the game that kicks off in Brisbane at the same time, likely by two or more goals, to avoid an embarrassing group stage exit at their home World Cup.
If Australia fail, they would be the first Matildas team since 2003 not to reach the knockouts, putting Gustavsson's job in real peril.
"I definitely think it's fair to say it's a failure if we don't go out of the group," Gustavsson said.
"I mean, everyone here understands that. Of course we want to go out of the group in a home World Cup, for sure.
"And I'm the first to admit that, I'll take ownership for that but that's not where we are (in terms of) mindset right now. Our mindset is just about playing the game in front of us."
The big question is what Kerr can, and will, do in that game.
On Sunday night, Kerr remained in the AAMI Park tunnel for the familiarisation session and again had a compression bandage on her left calf.
That morning, Australia's all-time leading goal scorer pulled the boots on late in the 15-minute training window accessible to media in the morning after completing her individual rehabilitation program.
When interviewed by Socceroos great Mark Schwarzer for host broadcasters Optus Sport, Gustavsson dodged multiple questions on whether Kerr had trained with her teammates.
At the press conference later that night in Melbourne, he again stressed a decision would come down to the wire.
"We're going to have a meeting tonight with the medical team and Sam and discuss what the possibilities are to play tomorrow. And then we're going to do some tests tomorrow to see what it looks like," Gustavsson told reporters.
"And I want to be honest here as well. Everyone here has been (involved) in sports, they know about calf injuries.
"One thing is that you're available but there's also risk when you come back from a muscle injury and I think everyone here understands that.
"So we need to balance how many minutes is she available? What are the risks? If we manage to go through to the playoffs, playing time and so forth.
"So there's a lot on the table to discuss, and we're probably won't know how exactly we're going to use that until we're coming into the stadium tomorrow."
Mary Fowler will return from concussion, pending final clearance, but Aivi Luik won't.
Kyah Simon, Gustavsson's gamble in his 23-player squad, also isn't available.
