While the British & Irish Lion's hamstring injury, sustained while playing for his club Northampton over the weekend, was bad news, England were still hopeful that Fin Smith would come through a practice session on Wednesday unscathed, leaving their key playmaker fit for Sunday's clash.
Mitchell got the bad news after undergoing a scan on the hamstring on Monday, learning that he'll miss all four of England's remaining games in the new international tournament.
It was the fourth hamstring injury the 29-year-old Mitchell has endured this year, leaving Jack van Poortvliet set to continue at No.9, with either Ben Spencer or Harry Randall in reserve on the bench.
But it's Mitchell's Northampton teammate Fin Smith who's now the focus of England concern as he faces a race to be fit for Sunday while battling with a calf injury that he picked up just over a week ago.
If he can't prove to coach Steve Borthwick that he's ready for the fray in Wednesday's training session at their Pennyhill Park base, either the experienced George Ford or Marcus Smith will replace him at flyhalf.
But Tommy Freeman, another of England's Northampton contingent, sounded an encouraging note on Tuesday, noting: "Fin is really good and he's tracking nicely."
Lions ace Freeman himself looks set to feature on the wing, with the Wallabies expecting him to be a key figure should England, as expected, launch the sort of high-ball bombardment that helped them prevail in last year's Twickenham fixture.
"It's an important part of the game," said Kurtley Beale at the Wallabies camp on Tuesday. "All teams use it to to their advantage. I feel like the Poms will be coming with that type of tactic again, no doubt.
"I saw that Freddie Steward's in the picture, you've got Freeman on the wings, they're very, very good, talented high-ball receivers. We'll be trying all our best to combat that and win those momentum swings."
With agencies
