The former England captain has been out since November when Liverpool striker Luis Suarez fell on his knee during the 1-1 draw at Stamford Bridge.
Terry was due to make his comeback against Sunderland on December 8, however a number of setbacks in training stopped him from doing so and continue to prevent him from making his return.
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“Talking to the doctor, the problem is a build-up of fluid on the bone. So at this stage we have no idea when he will be back.”
The Blues skipper is not the only absentee for Benitez, with Chelsea set to lose John Obi Mikel and Victor Moses to the forthcoming Africa Cup of Nations.
Goalkeeper Petr Cech is also sidelined for “two or three weeks” with injury and is expected to miss both legs of Chelsea’s Capital One Cup semi-final against Swansea.
However, striker Demba Ba is close to completing his transfer from Newcastle and Benitez is hopeful he could make his debut against Southampton on Saturday: He added: “If we can do the business I hope he can play in the FA Cup.”