Jonny Wilkinson is almost certain to miss England's opening Six Nations match against Scotland on 3 February, according to his fitness coach.
Wilkinson has worked with Black since he was 17
The 27-year-old was named in England's 33-man Six Nations squad on Tuesday.
But Newcastle fitness coach Steve Black told BBC Sport: "I don't think Jonny will be ready for the Scotland game.
"It would be foolhardy to rush him back. He needs time to adjust to the playing environment. We need to get him back onto the field for Newcastle."
With Wilkinson unlikely to play, either Toby Flood or the uncapped Shane Geraghty - neither of whom has started a game for England - will be selected at fly-half against Scotland by new head coach Brian Ashton.
Wilkinson has not played for the Falcons since suffering a lacerated kidney on 3 November against Bristol.
This latest setback was a pure freak injury
Newcastle fitness coach
He is joining in with fitness and skills sessions with his team-mates, but is yet to take part in full-contact sessions.
Wilkinson says he could make his comeback in the European Challenge games against Padova and Brive on 12 and 20 January "at best".
But the league matches with Leicester on 27 January or Bristol on 18 February look a more realistic target.
"I actually feel fine and ready to go, but my kidney injury is a
bit of a strange one," he told The Times.
"The issue here is the amount of precaution required before letting me loose completely."
Black, who has worked with Wilkinson since he was 17, says the fly-half is making excellent progress.
"I work one on one with Jonny every day and he's determined, ambitious and in great nick.
"He was very sore and uncomfortable following the injury, but the soreness has subsided now.
"A fit Jonny Wilkinson is a major bonus to the England squad."
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Wilkinson has not played for England since the 2003 World Cup final and has made only three starts for Newcastle this season.
He has been afflicted by injuries to his neck, shoulder, knee ligaments and kidney, but Black denies that Wilkinson will always be injury prone.
"This latest setback was a pure freak injury," Black said. "A whack to the kidneys is a boxing injury.
"It's a part of the body that's difficult to protect and is vulnerable, even for world champion boxers.
"Jonny is fit and strong and his recovery levels are high. And he's had a lot of practice at recovering."
Black also works with Flood at Newcastle and says the young fly-half has learnt a lot from Wilkinson.
"In rugby, as in life, you want to be with good, positive and ambitious people. Jonny Wilkinson would get a tick in all of those columns and Toby's certainly benefited from working with him."