Johnson out for at least a month
"Johnson has a problem, so he'll see a specialist and then we'll talk, but we know he'll be out for at least one month," Benitez told the club's website.
"But we are waiting for another opinion and then we'll know how long." Johnson, likely to be included in England's squad for next year's World Cup finals in South Africa, damaged the medial ligament of his right knee late in the 1-0 win at Villa Park.
Benitez hopes to have winger Albert Riera back within a week following a knock suffered against Blackburn Rovers on December 5.