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.”