Terry has not played for nearly a month because of a persistent knee injury and is rated as 50/50 by Villas-Boas for the first leg in southern Italy.
Villas-Boas has been under the microscope with Chelsea slipping out of the top four in the Premier League and being held at home by second tier Birmingham City in the FA Cup on Saturday and he was again forced to field questions about his future at a news conference in Naples on Monday.
A fit Terry would be a huge boost for the Portuguese and he will leave it as late as possible to decide whether the former England captain is fit enough to play.
"John is a player of great importance. I will leave it as late as possible and make a decision after training in Naples," Villas-Boas said.
"JT is a great leader. He gives you an extra emotional push but, at the moment, we've had to do without him.
"If he misses out, then we will miss a top player who gives us great defensive stability. It would be a massive loss."
Asked about his future he said he was carrying on with the "full confidence" of the club's owner Roman Abramovich.
However, he admitted Chelsea recent results had not helped dampen the speculation.
"With the results that we have been having the speculation is normal based on the cultural past of this football club," he said. "But you have to understand that sometimes there is different perspective."
Striker Didier Drogba denied on Monday that he had taken over the half-time team talk during Saturday's 1-1 draw with Birmingham at Stamford Bridge.
"I want to make things clear - the manager made the speech at half-time and after that I think he's got some leaders in the team," the 33-year-old Ivory Coast international said.
"We are here to help him. After that we were trying to bring the team up after half-time and nothing more.
"I think people are making a big thing about nothing."
Villas-Boas said the match against Napoli, who qualified for the last 16 at the expense of Premier League leaders Manchester City, was a massive challenge.
"Everyone saw what Napoli did in a group which put another English team out," he said. "The Champions League is a competition that everyone wants to win. The players will feel much more tension and feel it much more.
"There are two weeks between the games and it's important for us to be competent and to bring the game to Stamford Bridge with a win or a draw, ideally a scoring draw. Then we can finish the business here."
