An update on Lyon's status on Friday from Australia media manager Cole Hitchcock said the off-spinner would require "a period of rehabilitation after this match is concluded" but that any decision regarding his availability for the remainder of the series won't be made until the end of the Test.

Reacting to the news on commentary, Sky Sports' Michael Atherton said: "That doesn't sound hopeful at all.

"Often, when you get an update around injuries, they give a grade to the tear. That hasn't come with this, but it does say 'significant' calf strain - so it's clearly not just a little tear.

"Bad news for Nathan Lyon. Bad news for Australia."

Lyon arrived at Lord's on day three on crutches after having scans on the injury on Thursday night before further assessment on Friday. He is able to continue to play in the Test, most likely with the bat, if required.

Lyon pulled up in the field in Thursday's evening session and had to be helped off by a member of support staff. A cruel blow for the Australia off-spinner, who by playing at Lord's became the first specialist bowler to play 100 consecutive Test matches.

Australia batter Steve Smith said ahead of play: "Nath's not doing too well, so I think we'll struggle to see him out there.

"Hopefully I don't need to do a heap of bowling. [Off-spinner] Travis Head came out pretty well the other end, so we'll see how we go."

Speaking to Sky Sports, Sir Andrew Strauss said of Lyon's injury: "If you look at the teams side by side, he is the major difference. He gives Australia balance and is a threat no matter what the conditions. It would be a massive loss."

If Lyon's injury rules him out long term, Australia could call upon Todd Murphy for next week's third Test at Headingley and beyond.

Off-spinner Murphy is yet to feature in an Ashes Test but played four times in India earlier this year, taking 14 wickets at an average 25.21 with a best of 7-124 on debut in Nagpur.

Smith said: "Todd has been bowling beautifully in nets and bowled really well in India when he got his opportunity. I would be confident if he came in that he would do a terrific job for us."