The Croatian playmaker has been consistently linked with a move to Real Madrid this summer, but Villas-Boas said that Modric must be ready to play if a fee cannot be agreed.
Villas-Boas said: "He has been training separately. Now it is important for Luka to feel mentally ready to come back to first-team training.
"I think the player has to understand that the club cannot do anything about other clubs' offers.
"We have a value and an interest that we have to defend, and we are waiting to reintegrate the player if the offer is not matched."
Spurs continued their preparations for the new Premier League season with a 1-0 win against Watford on Sunday.