Mata's impending arrival is already being compared with the impact Mesut Ozil had on Arsenal in the last transfer window and United are being strongly tipped to bridge the gap, as well as become a stronger contender for the title next term.
But Mourinho is not worried and said: "We are not afraid of his going there to do very well for United. We want him to go there and be happy to do very well for United.
"He's a player who's been playing in the League for three years, so it's not a gamble to see if he adapts well. They are convinced he'll be important for them, so no fears.
"We hope he does well. We are convinced he's going to do well. If Man United pays their record for a player, it's because they know exactly what they want from him and what he can give them.
"We are convinced he's going to do well. So we wish that. Obviously, when we play him, we're going to try to win and he'll do the same, but the reality is that everybody in this club, the owner to the last man, wish him the very best."
For breaking stories and all the great banter like us on Facebook: facebook.com/tribalfootball