Willian looked set to be joining Tottenham, only for Mourinho to swoop to secure his services, although he revealed he had been aware of the Blues' pursuit for a while.
"They made an offer two months ago but I was not allowed to leave," he told Sportv.
"I kept following the club and I used to go to London to follow Chelsea whenever I had free time and now my dream has come true.
"I will work with Mourinho who is the best manager and I will grow a lot with him."
Willian will face stiff competition for a place in the team with the likes of Eden Hazard, Oscar, Juan Mata, Ramires, Andre Schurrle and Kevin De Bruyne amongst the names vying for a spot, something which he acknowledged.
"They are quality players and nobody can take their place for granted. So I have to work hard to get a place in the first team," he said.
"Other clubs were interested, but my intention and dream of playing for Chelsea made the difference.
"The opportunity to play with Mourinho, the best manager in the world, also made a lot of difference. I always dreamed of wearing Chelsea's jersey."
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