But the Socceroos say they expect Toure to return to training on Thursday (Friday AEST), though no specific reason was provided for his absence.
The striker, who has been widely expected to lead the line in Saturday's game at BC Place in Vancouver, wasn't involved in the 15-minute window of training media were permitted to watch on Wednesday.
Toure was the only player of the 26-strong squad not involved, while coach Tony Popovic and assistant Hayden Foxe were also spotted in discussion with team medical boss Mark Jones.
It would be a hammer-blow to the Socceroos if dynamic striker Toure is unavailable for any part of the tournament, given his importance to Australia's attack.
Defenders Jordan Bos and Milos Degenek spoke to the media after training - and in between the two press conferences, a Football Australia spokesperson said they were expecting Toure to train on Thursday.
"So Mo's expected to train tomorrow," Degenek said.
He quipped: "Answered that before you asked it. Mo is expected to train tomorrow.''
Asked if he knew why Toure didn't train, Degenek said: "You'll see him tomorrow."
Asked when the players noticed Toure wasn't in the session, Degenek said: "I didn't notice he wasn't out there, because a lot of the session was separate parts - attack (and) defence.
"But you'll see him tomorrow and I'm sure he'll be more than happy to speak to you guys."
Bos, speaking before Degenek, said he didn't know why Toure was absent.
"It was actually during training where I noticed he wasn't in there, so I don't know why he wasn't (there)," Bos said.
Toure, who has scored two international goals, started in Australia's recent friendly against Mexico, and was used off the bench against Switzerland last weekend.
The striker notched 10 goals, including two hat-tricks, and three assists in just 12 appearances for English Championship side Norwich after signing in January, despite a groin injury interrupting his season.
"He's a big asset to us," Bos said.
"He's been doing really well in his new club, so him scoring goals and his power and everything about him is a great asset for us."
Bos was asked how big a blow it would be if Toure wasn't available.
"I'm not too sure," Bos said.
"We have other players as well that can also play striker and stuff, but yeah, obviously to have Mo is really nice for us."
If Toure is not fit, one-cap striker Tete Yengi is the only other traditional No.9 on hand to replace him.
Veteran Mathew Leckie and explosive attacker Nestory Irankunda are other leading options.
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