The 2005 and 2009 Lion left the field after 17 minutes in the Scarlets 35-27 Heineken Cup triumph over Benetton Treviso having nudged his side into the lead with a penalty and conversion.
"It will take 24 to 48 hours to have an accurate assessment," said Scarlets coach Nigel Davies after the game.
"It's probably a tweak to the ligaments but we don't really know. He's walking around, so that's a good sign.
"In the next couple of days we'll have a clear assessment. We will have it scanned after the swelling has gone down."
To further the injury woes Davies revealed Lions hooker and Wales captain Matthew Rees was substituted after struggling with an abductor problem.