Newcastle's Jonny Wilkinson will be out for up to three months after suffering a "lacerated" kidney.
Wilkinson was playing his first game after an eight-week lay-off
The England fly-half picked up the injury in Friday's win over Bristol - his first start for eight weeks.
"It will be about two or three months, we're not putting any timescale on it," said Falcons coach John Fletcher.
"He definitely needs to be inactive for four weeks and then he starts his rehab. He'll be back when he's ready - it will be around eight to 12 weeks."
Wilkinson, who has been plagued by injury and has not represented England since the 2003 World Cup final, was playing his first game after two months out with a knee injury.
He played the full 80 minutes in the win over Bristol but now believes he suffered the injury just 20 minutes into the match.
It's so uncomfortable, training would have been out of the question
"Kidneys do heal quite quickly, but the specialist has seen the scans and says that I cannot do anything at all for at least a month," he told the Times.
"I must have taken a hit through the front of my stomach which smashed my kidney into a rib.
"I'm so disappointed. I can't really pinpoint the hit which did the damage. I think it happened about an hour from the end of the game.
"I felt it but was able to carry on. I don't think it really hindered the way I was playing.
"The annoying fact is that it seems that playing on may have made it a lot worse.
"I was in discomfort throughout the weekend. It was after that I had the scan, which showed quite a bad laceration.
"It's so uncomfortable, training would have been out of the question. I still can't walk without feeling it."
Wilkinson has suffered a long and varied series of injuries, including shoulder nerve, biceps, appendicitis, groin and, earlier this season, knee ligament problems.