Jamie Roberts and George North are both major injury doubts for the British and Irish Lions ahead of the first test against Australia in Brisbane on Saturday.
The Welsh pair are both nursing hamstring injuries, with Roberts being replaced against New South Wales Waratahs at the weekend while North missed the match in Sydney with an identical problem.
Lions team doctor James Robson says Roberts and North will be assessed further following Tuesday's tour match against ACT Brumbies - the final game before the start of the series - ahead of selection for the first test against Australia later this week.
"Jamie is a grave doubt for this week - there is no doubt about that," Robson said.
"You only had to see how much discomfort he was in at the weekend. However, further assessment will be made later in the week as to his availability going forward.
"George is an exceptional character and has worked really hard," Robson added. "He is back to running today. He is doing very, very well.
"An accurate assessment of his availability for selection will take place on Wednesday, but at the moment I think he is doing very well."
Roberts was expected to partner Brian O'Driscoll in the centre for the first test and is now likely to be replaced by fellow-Welsh centre Jonathan Davies while North, after scoring two tries on the tour so far, was first choice to begin on the left wing.
Sean Maitland is the primary candidate to replace North having been omitted from the 23 to face the Brumbies, with Wales' Alex Cuthbert likely to start on the right.
Coach Warren Gatland added on North: "His hamstring is still tight. We'll have to make the call on Wednesday.
"He has not trained. To be honest, he is less than 50-50."
Christian Wade and Brad Barritt were both called into the Lions squad as cover for the injured duo and will feature in this week's final warm-up game, but Tommy Bowe, who remains on the tour despite surgery on a hand injury, is unavailable.
Gatland added: "He's out of the cast. You can see where the wound is and it's healing well. At an outside chance he could be available for Saturday, but in all likelihood he'll be ready for the second Test."