Fernandes has joined Spurs on an initial 18-month loan deal with an option to making the transfer permanent.
The 21-year-old will provide vital cover in the middle of the park as Moussa Sissoko is out for three months with a knee injury.
Fernandes, who will wear the No.30 shirt at Spurs, said: “It’s one big dream to come to this big club.
“I will try to give my best for the club. Every day, every training session, every game. Every day I will try to be the best.
“The Premier League is a difficult competition because every player wants to be playing in it.
“I always dreamed of playing in the Premier League.”
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Mourinho will be hoping that more arrivals may follow, particularly a striker following Harry Kane’s potentially season-ending hamstring injury.
The club are understood to be interested in AC Milan’s Krzysztof Piatek while Christian Benteke, Edinson Cavani and even former striker Fernando Llorente have been linked.
Mourinho said last week that his side will never be ‘kings of the market’ and that this January will be a window of opportunity.
Fernandes was offered to them by Mourinho’s friend and super agent Jorges Mendes and they beat off competition from West Ham, who were keen on being the ones to bring him to London.
Given his age, Fernandes will be viewed as more of a signing for the future, but the Portuguese will at least add extra bodies to a squad which has been hit by injuries.
Fernandes has won two caps for Portugal and is a box-to-box midfielder in the ilk of Sissoko.
His parents live in the Tottenham area and he was recently spotted at his new club’s home game with Chelsea before Christmas.
There could also be outgoings at Spurs this month, with Kyle Walker-Peters hoping to join Crystal Palace on loan while the club are trying to offload Christian Eriksen this month in order to get a fee for him.