Nathan Lyon arrived at Lord's on crutches on Friday morning in a massive blow to Australia's Ashes hopes.
Lyon limped out of Thursday's afternoon session after suffering a calf injury while fielding. He looked distraught as he received treatment on the boundary, with team-mate Steve Smith admitting the off-spinner could be out of action for "a while".
Lyon needed crutches to join his international colleagues at Lord's on Friday morning, as he hobbled into the ground's iconic pavilion. His right calf was also covered in a protective sleeve. Australia gave an update on Lyon's fitness on Friday morning.
A spokesperson said: "Lyon has been diagnosed with a significant calf strain. He will require a period of rehabilitation after this match is concluded. A decision regarding his availability for the remainder of the series will be made at the conclusion of the game."
The 35-year-old is one of Australia's most important cricketers after playing 100 consecutive Tests for his country. His absence will be a boost for England, who are 1-0 down in this year's Ashes series with three-and-a-half Tests left to play.
Lyon made history on the first day of the second Ashes Test at Lord's on Wednesday when he became the first specialist bowler in history to play in 100 consecutive Tests. Only seven other players in the history of the game have matched that feat.
Lyon dismissed Zak Crawley for 48 on Thursday, but his historic Test took a turn for the worse when he pulled up while fielding. He needed a physio to help him off the field, forcing Australia captain Pat Cummins to bowl part-time spinner Travis Head.
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Smith gave an update on Lyon's fitness after Thursday's play. He said: "We're going to miss him for this game and could miss him for quite a while. It didn't look good. It would be a big loss for us... Fingers crossed he is OK."
A video clip of Lyon arriving at Lord's on Friday morning showed him being unable to put any weight on his injured right leg.
Todd Murphy, a 22-year-old off-spinner from Victoria, is likely to come in for Lyon for the third Ashes Test at Headingley, which starts in just six days' time. It will be difficult for Lyon to be fit again before the final Test starts at The Oval on July 27.
England also lost their frontline spinner just days before the start of this year's Ashes series when Jack Leach suffered a stress fracture, forcing Moeen Ali to end his two-year exile from Test cricket.
Lyon's absence will be a boost for England, who suffered a narrow two-wicket defeat to Australia in last week's first Test at Edgbaston. Ben Stokes and his players head into Friday's play on 278-4, still 138 runs behind Australia's first innings total of 416 all out.
England will need to win the remaining three Ashes Tests if they lose to Australia at Lord's to retain the Ashes. The hosts have not retained the Urn since beating Australia 3-2 in 2015. The Aussies won the last Ashes series 4-0 on home soil 18 months ago.