Modric has been training alone since being fined last month for going AWOL in an apparent attempt to force through a move to Real Madrid.
"He is training alone because we decided to do that until his mental state is ready to return to the team," Villas-Boas said.
"It is important that, in the coming days, he understands he must return to the first team.
"We respect the player's ambition, but he must also respect the club."
He added: "Until we receive a suitable offer for the player, he will not go.
"The contacts we have had with other clubs have not met with what Tottenham wants for the player.
"We respect the size of Madrid and the other interested clubs, but they must respect what we are doing too."