Mourinho saw his return to management at Spurs start off with a 3-2 win at West Ham on Saturday where Alli was a key player, even though Son Heung-min, Lucas Moura and Harry Kane grabbed the goals.
The Portuguese had taken time to talk to Alli following his appointment on Wednesday, after jokingly asking the 23-year-old if he was himself or his brother given his indifferent form over the last year.
Mourinho then told him to “play like Dele” and it looked to have the desired effect at the London Stadium as Alli was back to his best, notching an assist and a pre-assist with a brilliant piece of skill in the first half.
After flourishing under Mauricio Pochettino in his first couple of years at Spurs, Alli has had a dip over the last 12 months, culminating in him losing his place in the England squad.
But Mourinho believes he was back to his best against the Hammers.
“Dele was like the team, he was brilliant for 60-65 minutes and then he paid the price of the times, so he went a little bit down,” Mourinho said.
“He was the old Dele Alli, the Dele Alli of a couple of years ago, that impressed not just England, but the world.
“He did exactly what I wanted him to do, with two days of work.
“With important selection of information I tried to make clear for him exactly the spaces where we wanted him to play, offensively and defensively, he is an intelligent footballer to understand what we wanted and very, very important for the team.
“I am really pleased with his performance.”
His sublime piece of skill created his side’s second goal as, taking a loose pass from Eric Dier, he managed to keep the ball in, but having fallen over in the process, somehow managed to hook the ball to Son, who then set up Moura.
Mourinho added: “The piece if skill was amazing, but what for me is more amazing is the mental thing in relation to that.
“You only do that if you are focused, committed, ultra confident, if not you give up when you have bad feeling, sadness, you don’t do that piece of skill.
“It is amazing, but it only comes when from an emotional point of view everything is positive. Fantastic, I am really happy.”