The Blues captain was left out of the line-up for Saturday's FA Cup fifth-round tie against Birmingham, despite the knee injury which has sidelined him for three weeks improving.
However, Terry only returned to full training on Thursday and Villas-Boas has cast fresh doubt on whether the defender could figure in Italy.
Asked if he expected Terry to be fit for Tuesday, the Portuguese told ESPN: "We will have to wait and see. At moment there is a 50-50 chance of him making it.
"He's going to train on Monday and travel with the team. After that we'll make a decision."