The 25 year old had undergone a medical with Spurs, but could now sign for Chelsea after they entered the race for his signature on Thursday.
When asked whether the Brazilian had chosen to make the move to Stamford Bridge, Mourinho nodded.
"I know what the player wants, so in this moment we cannot hide. That is a possibility."
Willian has not yet agreed personal terms at the Blues. It is understood he is attracted by the chance to play Champions League football, compete for the Premier League title and work under Mourinho.
"I couldn't say much in previous news conferences," Mourinho said.
"I have to answer your questions and try to be honest, but at the same time try to hide a few things.
"I'm happy with my squad, but every one of us managers, we say we are happy, but we are always unhappy.
"We always want something of more quality and we never close the door to the possibility of adding more quality."