Newcastle Falcons fly-half Jonny Wilkinson had a scan on his right knee on Saturday amid fears he has sustained medial ligament damage.
The England World Cup winner left the field five minutes into the second half of Friday's 20-19 Guinness Premiership victory over Worcester Warriors.
The club will release a statement on Monday with further details of the injury once the scan results have been analysed.
The 27-year-old, who kicked England to World Cup glory in 2003 but has been beset by fitness problems since, picked up the injury early in the second half at Kingston Park when twisting his knee in a tackle.
Damage to the anterior-cruciate ligament is considered the worst knee injury for an athlete, but Wilkinson could still expect a spell on the sidelines before making a comeback.
Falcons director of rugby John Fletcher said ahead of the scan: "Indications at the moment are that it's his medial ligament, which is the lesser of the two evils.
"It was in a midfield contact and it looked like one of our players fell on his leg."
Wilkinson has suffered since the 2003 World Cup, including neck, bicep, knee ligament and groin problems. The only crumb of comfort this time 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.
Fletcher added: "It's a big blow. He's a world-class player in a key position."
England Under-18 head coach John Fletcher insists his young side will be better for the experience after beating Italy in the bronze final to round out their 2015 Rugby Europe U18 Championship campaign in style.