Owen Farrell faces a race against time to be fit for the British and Irish Lions' first Test against New Zealand after suffering a quadriceps strain.
The England fly-half will miss Saturday's Maori All Blacks clash with a "grade one" quad injury - and the Lions will hope he can recover in time to face New Zealand in Auckland on June 24.
Wales fly-half Dan Biggar will replace Farrell on the Lions' bench for Saturday's Maori clash in Rotorua.
"Owen Farrell has suffered a grade one quadriceps strain in training and has been ruled out of the British and Irish Lions match against the Maori All Blacks on Saturday evening," read a Lions statement.
"The England and Saracens playmaker will be replaced on the bench by Wales fly-half Dan Biggar."
Farrell had shown imperious form in the Lions' fine 12-3 win over much-vaunted Super Rugby leaders the Crusaders in Christchurch last weekend.
Head coach Warren Gatland will want the 25-year-old to play a pivotal role in the first Test against the All Blacks on Saturday week.
So the 52-cap England star will now launch an immediate bid to ease past this setback to contest selection for the Eden Park showdown.
Gatland hailed Johnny Sexton for rediscovering his "mojo" in naming the Ireland fly-half to start against the Maori.
Should Farrell fail to recover in time for the first Test, then Sexton would be almost certain to wear the Lions' number 10 jersey.
The Lions are already sweating on the full match readiness of tour captain Sam Warburton, who may yet not be in a position to start the first Test.
So Lions boss Gatland will be desperate not to have another of his blue-chip performers, in Farrell, entering the week of the first Test under a fitness cloud.
Ireland flanker Peter O'Mahony will captain the Lions against the Maori, with Warburton on the bench as he continues to shake off recent ankle trouble.
"Johnny needs more rugby and we're building on that," said Gatland of Sexton.
"He was really good off the bench against the Crusaders the other day and the combination of 10 and 12 was pretty seamless.
"He was just down a little on confidence but he's got a bit of his mojo back, and we wanted to give him a start against the Maori.
"Sam (Warburton) is well aware of the competition that is there at the moment.
"He fully understands that that loose forward trio (O'Mahony, Sean O'Brien and Taulupe Faletau) went outstandingly well against the Crusaders.
"The challenge for them is to repeat it. And if they do it again, we'll make what we think is the right selection for the Test match.
"One of the reasons we selected him as captain of the squad is that he is an absolute quality player.
"But this tour isn't about Sam Warburton, it's about putting the squad first.
"If he's not involved in the first Test because of the performance on Saturday night he will fully understand that."