Real Madrid are in talks with Spurs for the midfielder.
"He has been training separately. Now it is important for Luka to feel mentally ready to come back to first-team training," Villas-Boas told Sky Sports News after Sunday's 1-0 friendly victory over Watford, which Modric missed.
"These boys are at a different level in terms of their preparation, concentration and commitment.
"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."