Best, who scored Ireland's opening try and the third of his career, was replaced after 53 minutes by Jerry Flannery after picking up the knock. The injury has resulted in a small crack to the bone and the recovery period for this is between two and three weeks.
This will definitely rule Best out of selection for Ireland's final two pool games against France and Argentina.
The other injury news from the Ireland camp is that winger Denis Hickie suffered a bruised knee during the win over Georgia. The knee has responded to treatment today and although Hickie will not train tomorrow, he will not be ruled out of contention for selection against France.
Prop John John Hayes has a tight calf muscle following the game, but he will take a limited part in training tomorrow and is expected to be available for the France game.