Nathan Lyon is in doubt for the rest of the Ashes after the star spinner suffered a serious-looking calf injury on a disastrous day for Australia at Lord's.
England went to stumps on day two at 4-278 in reply to Australia's first-innings 416, with the game back in the balance after the hosts threw away three last-session wickets.
Friday morning now looms as critical, not only in the fight for a first-innings lead but as Australia await a clearer prognosis on Lyon's right leg.
The 36-year-old pulled up short when charging in to attempt a catch at deep backward square, immediately clutching at his calf afterwards and limping from the field.
The injury is likely to leave Australia without a frontline spinner for the rest of the match, with first-innings centurion Steve Smith and Travis Head sharing duties in the evening.
Beyond that, Todd Murphy is in Australia's squad for the series and would shape as the man called upon if Lyon is ruled out of the third Test at Headingley, starting next Thursday.
Such has been Lyon's longevity, with the Lord's Test his 100th in a row, Australia have not been forced to call on another tweaker to be chief spinner since 2013.
"Obviously it didn't look good," Smith said.
"It doesn't look ideal for the rest of the game. But yeah, I'm not sure how he actually is. But, you know, obviously if he's no good, it’s a big loss for us.
"He’s in his 100th consecutive Test match, which I know he was really looking forward to taking part in and having a role in as well."
Lyon's injury came after he took the sole wicket in England's innings until that point, outsmarting Zak Crawley on 48 and having him stumped down legside.
The wicket moved him to 496 in his career, and at that point it appeared as if the 500 milestone could come this week.
For all of Australia's pain for Lyon, England should have rammed home their advantage.
With Australia lacking their chief spinner and the score at 1-188, the hosts lost three men to short balls as they fell into an obvious trap from Australia's bowlers.
Ollie Pope, Ben Duckett and Joe Root were all caught playing the hook shot, with England's star batter Root having also been caught behind on one trying to hook a Cameron Green no-ball.
Pope toe-ended a Green bouncer to Smith backward-square on 42, Duckett hit Josh Hazlewood down Warner's throat on 98 and Root was neatly caught by Smith behind square off Mitchell Starc on 10.
Harry Brooks was also lucky not to fall to the tactic, dropped by Marnus Labuschagne on 25 off Pat Cummins.
He went to stumps unbeaten on 45, with Ben Stokes alongside him on 17 after taking multiple balls to the body rather than pulling.
"It was interesting," Smith said when asked if he was surprised England kept taking the short ball on.
"If they got under a few we might've stopped doing it maybe, but they kept taking it on and kept presenting opportunities for us."
Earlier, Smith had brought up his 12th Ashes century as Australia lost 5-77 on Thursday morning.
After resuming on 85, the vice-captain briefly appeared at risk of running out of partners when Alex Carey was trapped lbw by Stuart Broad and Starc edged off quickly.
But with Cummins (22no) at the other end, Smith brought up three-figures when he drove James Anderson through the covers, holding his arms aloft in celebration.
Smith was eventually caught in the slips driving trying to drive Josh Tongue (3-98), but his 110 drew him level with Steve Waugh for the second-most centuries by an Australian with 32.
