The 27-year-old, who withdrew from from the Three Lions' World Cup qualifying double header against San Marino and Montenegro, has not played for the Blues since damaging his meniscus against West Ham on March 17.
Cahill did not travel with the squad to Russia for the Europa League quarter-final second leg clash with Rubin Kazan and is expected to miss a further two weeks at a minimum.
"He had an operation," Benitez said. "We did it very quickly on the Saturday of the Southampton game [on March 30]."
The former Aston Villa and Bolton center back has made twenty-four appearances in the Premier League this campaign and Benitez will be hoping for a speedy recovery with fellow defender Ryan Bertrand recently joining Cahill and Ashley Cole on the treatment table.