The England international has not played since picking up the problem during the 2-0 win over West Ham United on March 17, and was forced to withdraw from the England squad to face San Marino and Montenegro.
It is believed that Cahill will miss the next two weeks of action, but Benitez remains upbeat about the situation.
"He had an operation," Benitez told Sky Sports News. "We did it very quickly on the Saturday of the Southampton game (30 March)."
Cahill will miss the Blues' Europa League clash with Rubin Kazan on Thursday.