The former England skipper injured his knee against Liverpool on November 11 and after initial fears suggested he may miss months, the diagnosis was lowered and he would be back playing within three weeks.
But Terry is yet to return to action and Benitez admits it is not clear-cut exactly when he will be back due to the complicated nature of the problem.
"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,” he said.
"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."