Joe Launchbury is a doubt to start England's opening Test against South Africa in Johannesburg due to a calf problem.
The Wasps captain sustained the injury in training last week and was unable to join the squad for their first significant practise session of the week at Kings Park Stadium in Durban on Tuesday.
He was poised to start the Ellis Park showdown on Saturday alongside Maro Itoje at lock but now faces a race against time to prove his fitness.
"Joe has just got a small niggle on his calf and we'll see where he gets to with that," assistant coach Neal Hatley said.
"It's hard to tell how much of a doubt he is, we'll have a look at what he's done this (Tuesday) morning. That will be ongoing."
England can ill-afford to lose a veteran of 52 caps who has been in strong form for Wasps when they seek to dispatch the Springboks at a venue Eddie Jones has described as the "spiritual home of rugby".
They departed for South Africa without injured locks Courtney Lawes and Charlie Ewels, leaving them stretched in a position that has been a traditional source of strength.
Launchbury's absence would see 20-year-old Nick Isiekwe partner Saracens colleague Itoje to make only his third Test appearance and uncapped Exeter lock Jonny Hill drafted on to the bench.
A left-field selection would be Kiwi Brad Shields making his England debut in the second row having joined up with the squad for the first time on Sunday.
Sam Simmonds missed the start of Tuesday's training session due to tightness arising from the flight over before eventually joining his team-mates.
Among the outstanding issues in team selection is the battle between tighthead props Harry Williams and Kyle Sinckler in the absence of Dan Cole, who has been rested for the tour.
"This is exactly what we want - two young tightheads fighting for that spot," Hatley said.
"Both of them are fairly explosive blokes. They're good scrummagers. You saw the job Kyle did against France, maybe his point of difference is his ball carrying.
"He's done well carrying the ball against France and Ireland. Harry is a big man who hits hard, cleans out well and is a good defender. They have different strengths."