Fiji had looked on course for one of the biggest upsets in tournament history on Saturday as they held a nine-point second-half lead over the Wallabies before the favourites tightened up and won the Pool D clash 39-21 in Sapporo.

To make matters worse, the Pacific islanders lost rampaging loose forward Peceli Yato to concussion for the Uruguay match in Kamaishi.

Yato was felled in a controversial tackle by Wallabies winger Reece Hodge, who appeared to make contact with the flanker's head in a no-arms tackle in about the 26th minute.

Clermont player Yato had been instrumental in Fiji's blistering start with a try and several surging runs, but failed his head injury assessment (HIA) and did not return.

While the Fijians protested the tackle during the game, match officials deemed there was no foul play, although pundits have criticised it.

Yato's fellow loose forward Viliame Mata was another injury concern for Wednesday's meeting against Uruguay, McKee said.

"With a short turnaround, Peceli can't be right for the next game because it is a six-day turnaround for an HIA," McKee said, referring to the protocol regarding return to play after a head injury.

"On Viliame, we will just have to look how he responds in next 24 hours."

McKee had set his team the target of becoming the first Pacific Islands side to make the knockout phase since 2007 and winning the Uruguay match was now crucial to those plans.

"It's a dangerous match for us," he said.

"The boys will be pretty tired. We've got some travel (on Sunday) and on Monday we'll be focusing on that Wednesday game, it's a very critical game for us."

Australian Associated Press