British media reported the Anzhi Makhachkala forward was set to join Tottenham in a 30 million pounds deal after a medical at the north London club amid interest from Liverpool.
Chelsea, however, have entered the race to sign the player and manager Mourinho struck an upbeat note when discussing the potential move ahead on their match against Manchester United on Monday.
"I think he's already made his decision. I don't like to speak before time because football can betray you," Mourinho told reporters.
"I could not say much in previous press conferences. I have to try to be honest and try to hide a few things.
"If you remember, I said I am happy with my squad but every manager says we are happy but we always want something more and we never close the door."
Willian joined Anzhi from Shakhtar Donetsk in January, but the club's owner Suleiman Kerimov has tightened the purse-strings at the Dagestan club, with the entire first-team available for sale, according to media reports.
Capped twice by Brazil, Willian can play as an attacking midfielder or out wide.
"That's the danger of medicals before contracts," Mourinho said having seemingly got one over his former protege and current Tottenham manager Andre Villas-Boas, with whom he worked Porto, Chelsea and Inter Milan
"The best thing to do is do the medical in secret."
When asked if the deal was close, Mourinho joked: "We have to do a medical."
The Portuguese also quashed speculation that Spaniard Juan Mata could be about to leave Stamford Bridge.
"No speculation," he said. "He is a player we all like, we all want to keep and we want more quality in the team, not less quality.
"We want to add quality to that quality we have. He isn't a player we want to lose."
Chelsea could allow some players to leave on loan, however, as Mourinho is wary of working with an unwieldy squad.
"Do we need to sell?" he asked. "No. Do we need to loan? Probably yes, because I don't like to work with big squads. 22 is the perfect number."
Chelsea travel to Old Trafford to play United on Monday and Mourinho said he would miss the relationship he used to enjoy with their former manager Alex Ferguson, who retired at the end of last season.
"For me, the difference is the routine of meeting a friend, of being together and relaxed before the match and have the guarantee that after the match, independent of the result, we will be together and enjoying our time and having a laugh," he said.
"With (new United manager) David (Moyes), we don't have this type of relationship but we have a good one, so we can start this kind of very nice relationship like I had with Sir Alex no problem.
"It is not manager v manager, but club v club, 11 v 11, big player v big player. We will just do our job."
(Reporting by Toby Davis; Additional reporting by Josh Reich; Editing by John Mehaffey)
