The Scarlets say their Wales wing George North suffered "no significant or serious" damage to his neck in Friday night's RaboDirect PRO12 clash against Ulster.
North was carried off with his neck in a brace and taken to hospital, but he was able to fly home from Belfast with his Scarlets colleagues on Saturday.
North faces further assessment from the Scarlets medical team on Monday. At this stage, it is not known how long he could be sidelined for.
Wales begin their RBS 6 Nations title defence against Ireland in Cardiff on February 2.
They have already lost fly-half Rhys Priestland and lock Luke Charteris from the entire Six Nations tournament because of injuries.
In a statement, the Scarlets said: "The Scarlets would like to express their gratitude for the concern raised by supporters and colleagues in the rugby world regarding George North following the neck injury he sustained against Ulster on Friday evening at Ravenhill.
"He was taken to hospital after the incident where, as a precaution, his neck was both X-rayed and scanned.
"The results from the two scans showed that there was no significant or serious damage to his neck.
"George travelled home with the rest of his Scarlets team-mates this morning on a flight from Belfast.
"He will be assessed further by the Scarlets medical team on Monday. It is too early to assess the full extent of the injury, or indeed, the recovery period at this stage."