England pace bowler James Anderson is a doubt for the first Test against South Africa starting on 26 December.
Anderson, England's leading wicket-taker in Tests, was sent for scans on Tuesday after suffering tightness in his right calf muscle.
A decision on his involvement in the first Test in Durban is unlikely to be taken until Thursday.
"It's a concern," said coach Trevor Bayliss ahead of the four-match series against the top-ranked Test side.
"He hasn't been able to bowl a lot of overs on this tour. He has bowled overs, though, so whatever it is it's not a terribly bad one."
The 33-year-old Anderson was rested for England's victory over South Africa A in their final warm-up game.
Having experienced soreness in his calf since the start of the tour, he has bowled only five overs, in a 13-a-side match against a South Africa Invitational XI last week.
Anderson, with 426 wickets in 110 Tests, missed the final two Tests of England's Ashes triumph over Australia this summer with a side strain.
If Anderson is ruled out, England are likely to choose two from Steven Finn, Chris Woakes, Chris Jordan or Mark Footitt to join Stuart Broad in the pace attack.
|South Africa Test series dates|
|26-30 Dec: 1st Test, Durban||14-18 Jan: 3rd Test, Johannesburg|
|2-6 Jan: 2nd Test, Cape Town||22-26 Jan: 4th Test, Centurion|
Finn, 26, is likely to get the nod having claimed six wickets in England's win by an innings and 91 runs over South Africa A, despite being a late call-up to the squad after recovering from a foot injury.
"What he has done over the last 12 months for the Test team has been fantastic," Bayliss noted. "He is a great back-up to Broad and Anderson. If he is fit, he would be difficult to leave out."
South Africa paceman Dale Steyn, the number one-ranked Test bowler, has been declared fit to face England after missing the recent 3-0 series defeat in India with a groin strain.
Fellow bowler Vernon Philander is expected to miss the start of the series after injuring his ankle playing football in India in November.
AB de Villiers will keep wicket for the hosts in at least the first two Tests after Dane Vilas was dropped and Quinton de Kock was not included in their 13-man squad.