Brazil international Willian enjoyed his finest individual campaign at Stamford Bridge last season, though his team-mates were unable to follow suit and their Premier League title defence ended in tatters, with Mourinho sacked in December.
Mourinho is poised to take charge at United following the sacking of Louis van Gaal and has reportedly earmarked Willian for a £60 million swoop when he assumes control.
And Willian was keen to talk up his relationship with 'The Special One' as he prepares for the Copa America Centenario.
"Jose is one of the most successful coaches in the world and Manchester United will be stronger with him as manager," the winger, 27, is quoted by The Sun.
"I played some of my best football under Mourinho. He is a special coach and I have a great relationship with him.
"Winning the Premier League with him was one of the highlights of my career. I was sad things didn’t work out after that."
Willian scored 11 goals in all competitions for Chelsea in 2015-16.