Cahill will learn on Saturday whether he'll travel to Honduras for Australia's do-or-die 2018 FIFA World Cup qualifier after sustaining the injury in Melbourne City's 1-0 defeat to Sydney FC on Friday.
Cahill was forced off in the first half at AAMI Park after landing awkwardly when attempting to hurdle Sky Blues defender Jordi Buijs and whisked away to hospital for scans.
Despite some swelling, the 37-year-old was able to walk and put weight on the ankle, suggesting the issue is a minor one.
But with the Socceroos' record goalscorer due to leave for Central America on Sunday with a 30-hour journey involving three flights for the match in San Pedro Sula, doubts now surround his availability for the first leg.
Sydney FC coach Graham Arnold, who coached Cahill as Socceroos boss a decade ago, has suggested Australian fans should say a little prayer.
"Lets all keep our fingers crossed he's okay," Arnold said.
"I just want us to qualify and we need our best team out there.
"The first thing I thought of straight away was the national team.
"Timmy's our star.
"You can't give (enough) credit for someone who does so much for the country.
"Against Syria, (scoring twice) over 120 minutes, what he did was phenomenal.
"I'm sure he's OK. He'll be ready at least for the return leg here in Australia."
The home-and-away tie with Honduras concludes in Sydney on November 15.
Melbourne City coach Joyce defended his decision to give Cahill, who was carrying no injuries into the match, his first start of the season.
"Timmy's desperate to play. He's been training hard. It's a big game. We're trying to win a big game. That's it," he said.
"The fact the national team coach (might have wanted him to rest) has got no bearing on it."
Told the 37-year-old had a swollen ankle and was off for scans, Arnold suggested tackling the long transit across the Pacific Ocean was a no-go.
"Once he gets on the plane it will swell up so he's better off staying here for the second leg," Arnold said.
"Let's all keep our fingers crossed that he's OK."
