Sean O'Brien will miss Leinster's Guinness PRO12 play-off clash against the Scarlets on Friday night.
Leinster said that the Ireland international flanker was suffering from tightness in his calf and a decision was taken "not to risk the injury further."
O'Brien, who was due to have been among the Leinster replacements, is the second injury concern this week for Lions head coach Warren Gatland.
Wales hooker Ken Owens had been due to lead the Scarlets against Leinster, but he suffered an ankle injury during training with his region earlier this week.
Owens has already been assessed by the Lions, and further checks are set to be carried out on him ahead of next week's pre-tour training camp in Ireland.
Lions players not involved in domestic and European play-off finals next week will assemble on Sunday for the Ireland get-together.
The Lions then leave for Auckland on May 29, with their opening tour game being against a New Zealand Provincial Barbarians XV five days later.