Nathan Lyon has been diagnosed with a "significant" calf strain after he limped off the field during the final session of the second day at Lord's leaving him highly doubtful to feature again in the series.
Chasing a ball towards the boundary in the 37th over, Lyon pulled up and was immediately in pain. He hobbled to the edge of the field where the physio began to assess him then had to be helped around the rope to the pavilion.
He was assessed overnight and arrived at the ground on crutches on Friday. It's understood he may bat later in the game if required although runners are no longer allowed in international cricket.
"He will require a period of rehabilitation after this match is concluded," a CA spokesperson said. "A decision regarding his availability for the remainder of the series will be made at the conclusion of the game."
It is a cruel twist for Lyon who is playing his 100th consecutive Test - the first specialist bowler to reach the milestone - and was lauded before the match for his durability in having played unbroken since midway through the 2013 Ashes.
Lyon had already shaped as a significant figure in this match - as he was at Edgbaston - having removed Zak Crawley to break an opening stand of 91, drawing him down the pitch for a stumping that was expertly completed by Alex Carey.
Ben Duckett, who led England's batting effort on the second day, expected Lyon to play a key role and questioned the impact his absence would have on the rest of the attack after the quicks opted for a short-ball approach.
"It's a huge shame and I really hope it's not too bad for him," he said. "You never want to see anyone go down with an injury. We all wish him the best. He was going to play a massive part in that fourth innings, he's such a good bowler. It will be interesting to see how they go about it.
"If they keep going bumpers with all four bowlers, I think they'll be quite tired by the end of it, especially with back to back Tests. Travis [Head] bowled all right tonight, so he maybe he will bowl a bit."
Lyon's absence leaves Australia having to rely on Steven Smith, Travis Head and Marnus Labuschagne for any spin required.
It is the second time in consecutive Lord's Tests Australia have suffered a major blow after Smith was concussed in 2019. On that occasion he was able to replaced under the protocols by Labuschagne, but this time the team will have make do.
Looking further head into the series it will bring Todd Murphy, the 22-year-old offspinner, into the frame for when the series reaches Headingley. Murphy took seven wickets on his debut against India in Nagpur earlier this year and claimed Virat Kohli four times in the four matches.
"He [Murphy] has been bowling beautifully in the nets and bowled really well in India when he got his opportunity as well," Smith said. "I'd be confident if he came in he'd do a terrific job for us, but fingers crossed Nathan's all right."
Legspinner Mitchell Swepson is now also in the UK having recently signed to play the County Championship for Glamorgan.