Spurs appeared poised to announce a deal for the Anzhi Makhachkala forward after he completed a medical, but Chelsea have now hijacked the deal after making their long-standing interest known.
A deal was quickly agreed between the Blues and the Russian club - respective owners Roman Abramovich and Suleyman Kerimov are friends - and Willian has been keen to move to Stamford Bridge since first becoming aware of a potential switch to Chelsea two years ago.
He will also have the prospect of Champions League football, something Spurs were unable to offer.
"I think he's already made his decision," Mourinho said.
The self-styled Special One nodded in the affirmative when asked if that was Chelsea.
He added: "I don't like to speak before time because football can be crazy.
"I know what the player wants, so in this moment we cannot hide, that it's a possibility."
Willian is still to undergo a medical at Stamford Bridge, but if he signs it strikes a blow for Mourinho over his former staff member Andre Villas-Boas, once of Chelsea and now Tottenham boss.
It could also hinder the prospective sale of Gareth Bale to Real Madrid, Mourinho's former club.
Source: PA
Source: PA