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Priestland impressed in the 51-3 victory over the Scots, coming off the bench before suffering the injury at the Millennium Stadium.
But with 30-cap Priestland back and fit for Scarlets' game against Connacht on Sunday, Easterby admits his return could prove crucial.
"It is great to have him back. When he is playing well and on top of his game there are not many better," Easterby told Wales Online.
"The likes of Rhys and Jonathan (Davies) have a lot of responsibility in terms of performance."