Chelsea captain John Terry will be out for a further four weeks after it was confirmed he would have knee surgery.
The England international, who has already been absent for the previous five weeks, missed the Champions League last 16 first leg defeat to Napoli on Tuesday.
Terry is set to miss the FA Cup fifth round replay against Birmingham City and the return leg against Napoli in the Champions League, with a return in the London derby against Tottenham Hotspur on March 24 potentially in the offing.
"I think four weeks would be excellent," Villas-Boas said. "We will have to wait to see tomorrow how the operation goes and hopefully the player can return in that frame of time.
"Normally it's four or six weeks. Without John we are missing a very important player for the team and have suffered a lot of goals recently, contrary to what was happening at the beginning of January.
"We have full belief in these players and have to continue to work to get it right [but] I think we have to focus on reorganising this relationship between the back four.
"We have to solve these mistakes at the back so that you can profit better from the opportunities you have and the goals you score."
Terry will also miss England's friendly with Holland, the first international since being stripped of the captaincy for a second time by The Football Association.
Interim manager Stuart Pearce is likely to chose a young, inexperienced squad for next week's friendly, with Terry likely to have been left out regardless of his fitness.
However, his latest injury reportedly makes him a fitness doubt for the summer's European Championship finals, and given the recent off the field headlines, his selection is far from a forgone conclusion.