Villas-Boas says he has been given assurances by chairman Daniel Levy that the 23-year-old will not be sold.
"The chairman said that we are not open to offers, whatever they are, and that the player will stay," he told O Jogo.
"In football anything can happen, but these are the guarantees given to me.
"There was a moment last season when Gareth came to me because he was not comfortable out on the pitch.
"I do not know if it was my response or that he was glad to relieve the stress, but after that he became confident and played with freedom.
"He felt important and finished the year in style."