North Wales' Wales wing George North was carried off on a stretcher during the Scarlets' 47-17 Pro12 defeat by Ulster at Ravenhill on Friday night.
The injury gives Wales a worry ahead of the Six Nations in which they kick off against Ireland in Cardiff on Saturday, 2 February.
Scarlets backs coach Mark Jones said: "I only know what I saw on the replay.
"He's had a knee to the head and he's struggling with some head and neck issues."
North, 20, has scored 11 Wales tries in 26 appearances, including two on his 2010 debut against South Africa.
He joins fellow 2012 Grand Slam winning back and team-mate Rhys Priestland on the injury list.
Fly-half Priestland is out of the 2013 campaign because of a torn Achilles and is a doubt for the British and Irish Lions tour next summer.
However, centre Jamie Roberts returns from injury for Cardiff Blues in the Pro12 at Munster on Saturday.
And Leigh Halfpenny has recovered from a neck injury to take a place on the bench for the Blues.
Eight Wales forwards are also currently sidelined - props Craig Mitchell and Aaron Jarvis, hookers Huw Bennett and Ken Owens, locks Alun Wyn Jones, Ian Evans and Luke Charteris as well as flanker Dan Lydiate.
Wales' injured players
•Craig Mitchell - prop
•Aaron Jarvis - prop
•Huw Bennett - hooker
•Ken Owens - hooker
•Alun Wyn Jones - lock
•Ian Evans - lock
•Luke Charteris - lock
•Dan Lydiate - flanker
•James Hook - fly-half
•Rhys Priestland - fly-half
•George North - wing
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