Interim manager Benitez was hopeful Terry would be fit to return for one of the Blues' four matches over Christmas and the new year.
But he stressed that he could not "guarantee" that, adding of his captain's recovery: "It's one step forward, one step back."
Benitez confirmed Terry, who had been scheduled to return almost two weeks ago, would definitely miss Sunday's Barclays Premier League game against Aston Villa.
Terry trained without the ball on Friday but was still not ready to return to full-contact exercises, with Benitez revealing the defender was prone to an adverse reaction 24 hours after training.
"The knee is quite complicated," he said.
"At least he's now on the pitch - that is positive news - but we cannot say, we cannot predict how long it will take."
Asked if Terry would make his comeback in the next fortnight, Benitez said: "I hope so but I am not a doctor.
"So I cannot guarantee anything."
Benitez was confident Terry would not require further surgery and would not be drawn on reports the 32-year-old was close to opening talks on a new contract to replace the one that expires in 18 months.
Pressed on Terry's importance to Chelsea - there is a huge difference in their win-loss record with and without their skipper - Benitez said: "If he's the captain, it's because he's a player that can be a leader.
"I have seen he has a great character."
At least Chelsea have cover for Terry in defence.
The same cannot be said for their attack if they choose to sell Daniel Sturridge to Liverpool when the transfer window reopens.
A fee has reportedly been agreed for the England striker, with talks said to have reached an impasse over agents' fees.
Benitez would only discuss Sturridge's fitness today, confirming the 23-year-old was ready to return from the hamstring problem that has kept him out for more than a month.
"The situation with Sturridge is the same," Benitez said.
"Now he's training, he's had another training session.
"He's a player that is available.
"I have to decide if he will be in the squad or in the team."
Benitez was similarly coy about Chelsea's own January transfer targets, although he admitted he had held talks with the club's board over new players.
"Everybody knows more or less what we are looking for or what we'll try to find," said Benitez, who will try to sign at least a striker and possibly a midfielder next month.
The Spaniard must also decide whether to recall any of Chelsea's on-loan players, principally Romelu Lukaku from West Brom.
Asked if that was an option, he said: "I cannot say no.
"But I cannot say yes.
"It's something that we have to analyse in January."
He added: "I am talking with the people in the club about how they were doing and we have some information, but no more."
With Victor Moses and John Obi Mikel expected to depart for African Nations Cup duty early next month, Benitez admitted it was important Chelsea got their transfer business done sooner rather than later.
He also conceded it was vital they produced a successful festive programme as they eye the daunting task of eating into the 13-point gap to Premier League leaders Manchester United.
"Always, it's time where you will see where you are," Benitez said.
"Because you play so many games, it's a crucial time."
It could also prove important in Benitez's attempts to further quell the fan revolt against his appointment.
Four wins from five games have certainly not done him any harm and he said: "At the end of the day, you have to judge the manager for the things that you can see on the pitch.
"The last games, we were doing well. Hopefully, we can be consistent and - always I say the same - it will easier.
"I will be working hard as always and I will continue working as hard as I can."