Lyon appeared to hold his calf muscle after running in from the boundary Thursday to field a shot from England’s Ben Duckett.
The 35-year-old slowly hobbled off the field and sat down for several minutes while a member of Australia’s medical team checked his injury.
Appearing to wipe away tears, Lyon eventually limped around the boundary edge towards the dressing room while a staff member held on to stop the bowler losing his balance.
“Nathan Lyon suffered an injury to his right calf while fielding in the final session today,” a Cricket Australia spokesman said.
“He will be further assessed after play. A further update, if available, will be shared in the morning.” Lyon’s absence from the rest of the second day and potentially much longer is a huge setback for Australia’s bid to retain the Ashes. Australia’s star batsman Steve Smith, who hit 110 on Thursday, conceded Lyon’s injury could prove a turning point in the Test.
“I don’t know how he is. It didn’t look good. Doesn’t look ideal for the rest of the game, losing a spinner on a surface that isn’t offering a lot to the fast bowlers,” he said.
“If he’s no good (to play) it’s a big loss for us. Fingers crossed he’s ok. He’s in his 100th consecutive Test and I know he was looking forward to it.”
Australia are 1-0 up after winning the first Test of the five-match series at Edgbaston.
Earlier on Thursday, Lyon had dismissed Zak Crawley to claim his 496th Test wicket and ninth of the series.
Lyon’s appearance at Lord’s made him only the sixth man to play in 100 consecutive Tests and the third Australian after Allan Border and Mark Waugh.
His 496 Test wickets place him eighth on the all-time list and fourth among spinners.
Shane Warne and Glenn McGrath are the only Australians to have taken more Test wickets than Lyon.