Terry suffered the injury on his comeback from suspension in the Blues' Premier League clash with Liverpool on November 11, but had been predicted to return within three weeks.
However, Benitez has conceded the former England international is struggling to get fully fit, telling reporters: "The knee is quite complicated. Sometimes it's really good, it's going really well and the day after we have to wait a little bit and we start again.
"I think obviously it will be closer every day but you ask me the question will it be closer [and] I cannot say how close."
When asked if there is a date set for Terry’s return, he responded: "No. We can say that it is getting better and this morning I was talking with him and he told me this, that he was fine and then the day after he was feeling a little bit worse, and then two days after he was fine, so we've got to see."
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He added: "The players are okay, we have suspensions but in terms of fitness they are fine and available, apart from what we knew before. Daniel Sturridge was training with us, he is fine."
The 52-year-old takes his side to face Leeds United in the quarter-finals of the Capital One Cup on Wednesday and is hoping a lack of preparation will not impact on their performance.
He added: "We played seven - with this one it will be eight - games in 24 days, so it's not easy especially when you have a new manager. You don't have time to prepare games and you have to play and go to Japan and come back so it's quite difficult.
"But that's the situation and we have to manage the situation. We played against Corinthians in Japan and they were waiting for us for 15 days and we were just playing a game, going there and we have to play quickly another game.
"When you are in a top side you are in different competitions. That is the way."