First winger George North was carried off the pitch on a stretcher during the Pro12 clash between the Scarlets and Ulster.
His injury gives Wales a worry ahead of their Six Nations fixture against Ireland next month.
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Sun and showers this afternoon with patchy frost on the way this evening.
Plaid have been handed an election boost as influential Labour supporters urge people to pick the Welsh nationalists on regional ballots.
A record crowd of more than 66,000 is expected at the Principality Stadium today for the back-to-back Welsh derbies.