Full-back George Furbank is a doubt for the start of England's Six Nations campaign after fracturing his arm in Northampton's Champions Cup win over the Bulls on Saturday.
The 28-year-old was removed 29 minutes into Saints' 30-21 victory after being tackled by Bulls skipper Elrigh Louw.
Furbank was being assessed on Tuesday and may need an operation.
"He has fractured his arm, it can vary so nothing concrete on how long that will take," said Northampton head coach Sam Vesty.
"We are not 100% sure what the outcome of that medical assessment will be."
Setting an accurate estimate for Furbank's return is complicated by recovery timescales differing between types of fracture and different patients.
However, with England's Six Nations opener away to defending champions Ireland on 1 February only six and a half weeks away, a key part of England's back division could well be absent for the first round.
Furbank started three of England's four autumn internationals having supplanted Freddie Steward halfway through the 2024 Six Nations.
Steward's high-ball ability was preferred by Borthwick for November's 29-20 defeat by South Africa and the Leicester man is the likeliest candidate to fill in for Furbank once more at Test level.
Furbank's absence will be keenly felt by Saints who hope that the win in Pretoria can kick-start their form amid a faltering Premiership title defence.
They lie eighth with three wins from seven matches before their away match against Saracens on Sunday.
"It is a big loss, he is a fantastic rugby player," added Vesty of Furbank.
"He is our club captain and has really grown into that leadership role over the last couple of years. He will be a loss.
"His counter-attacking ability, his ability to make other people look very good, his calmness are all real strong traits of George's and have got him a long way."