Jonny Wilkinson will see specialists on Saturday after twisting his right knee during Newcastle's dramatic victory over Worcester at Kingston Park on Friday night.
Wilkinson was helped from the field early in the second half after being caught under a team-mate at the bottom of a ruck.
Early assessments suggested he had suffered medial ligament damage but Newcastle director of rugby John Fletcher said: "I don't think it's too serious.
"It's too early to tell how bad it is. He got a twist and a bang on the knee when one of our players fell on him in the ruck.
"It's a big blow. He's a world-class player in a key position."
Wilkinson has suffered a catalogue of injuries since the 2003 World Cup, including neck, bicep, knee ligament and groin problems.
The only crumb of comfort last night was that he has not hurt the same knee he damaged twice before the 2005 Lions tour.
Even if it is not serious, the injury is a blow to Wilkinson's hopes of a recall to the England squad for the autumn internationals.
Only this week England's new director of elite rugby Rob Andrew warned those with international aspirations they must "deliver in the next six or seven weeks the form you would expect to have to produce if you want to get picked to play for England".
Without Wilkinson, Newcastle went on to complete a last-gasp 20-19 Guinness Premiership victory over the Warriors.
The Dutchman Tim Visser, who came on for Wilkinson, scored the decisive try in the last minute of the game.
Newcastle have massive hopes for Visser, who was spotted during a sevens tournament in Holland and brought to England where he joined Barnard Castle school and the Falcons academy.
"Tim's a talented bloke. He can play generally anywhere. He did well for us on the wing," said Fletcher.
At Adams Park, Wasps proved once again proved there is more to their game than "razzle-dazzle" as they dug out a 23-17 victory over London Irish.
The Exiles dominated the line-out, Wasps held the edge in the scrum but the game hardly opened up at all.
The highlight of the evening was Josh Lewsey's try just before half-time.
The England international gathered a bouncing ball, chipped expertly around Delon Armitage and scored in the corner to give Wasps a 14-8 half-time lead.
Topsy Ojo had earlier scored for London Irish but the game was otherwise a tale of the kickers, with Alex King and Jeremy Staunton booting 18 points between them for Wasps and Riki Flutey 12 for London Irish.
"Last Saturday against Saracens and the last half hour tonight was a Wasps performance we couldn't put in last year, where we just squeeze the life out of the opposition," said Wasps coach Shaun Edwards.
"If we are going to win trophies we can't be one dimensional. Last year we had a lot of razzle-dazzle but the tight game wasn't there.
"Hopefully this year, with the signings we've made and the work we've done in the off-season, we'll have the tight game to dominate proceedings.
"You have got to have both those styles in your armoury."