The former Chelsea, Manchester United, Real Madrid and Tottenham boss will be part of talkSPORT's Euros coverage this summer and joined the Breakfast show as the tournament prepares to kick off tonight.
The Special One was eager to stress he fully supports Southgate and the England players, many of whom he knows well, but gave his honest opinion on the Three Lions and how they should line up in their opening game.
Intriguingly, he would find no place for Luke Shaw or Marcus Rashford in his team, despite working with them at Manchester United.
He said Southgate should play a back four and unleash his attacking talents and declared Grealish simply has to start as he compared the Aston Villa playmaker to Portugal legend Luis Figo.
Here's everything he had to say about England ahead of the clash with Croatia on Sunday…
Mourinho spoke at length about just how good their midfield is and believes this is the vital area of the game.
The Portuguese knows Luka Modric well having coached him at Real Madrid, and is a huge fan of Marcelo Brozović and Matteo Kovacic.
"Looking to the first match against Croatia, and I think this is a tournament where you have to go match after match and forget what is coming after," he said.
"England is going to face one of the best midfields in the tournament. Brozovic is the top of the top. He's a fantastic player. Aggression but at the same time great technique and arrival in the box and coming from a more defensive position.
"Modric has had a fantastic career. Kovacic is better than what people say. He's played in every club I've been but never with me. He played for Real Madrid, but not with me, he played for Chelsea, but not with me and played for Inter but not with me.
"I love the guy and he's following my clubs.
"When you play against a strong midfield, it's there you have to control. Why to play with three at the back when their power is not there.
"Since Manduzkic finished where is the top striker for Croatia? It's their problem. They have Perisic on the side, but is not a striker. Rebic plays around but it's there in midfield.
"For me, you play with two centre-backs and concentrate on your midfield.
"I think to be offensive, you have to be in control of matches. How can you be offensive if you don't have the ball? It is important to control that midfield and it's more important to play with three midfielders.
"It's important to be in control of that area. I think it's enough to play with a back four and the full backs with the freedom to go.
"When I say three midfielders, one of them can be, for example, Phil Foden. It's not like we are speaking about Henderson, Rice and Kalvin Phillips.
"I believe it's important to be in control. You let them play, they play. They play and take the initiative from you.
"Emotionally it's not good because you are at home and you want to start. It's important to dominate and don't let them play. Dominate is to be solid and be in control. When you have a good midfield like England can have and then in attack, you are adding three attacking players to this midfield.
"I think England has everything. Gareth has tried many times with five at the back, so I don't see a problem if he does want to go with that."
England's record at the European Championship
Mourinho was presented with the incredible stat that England have never won their opening game of a European Championship.
He said: "You English are always pessimistic and you look to the negative stats. I prefer not to look at it when it's negative. Just forget it.
"You have the opening match against Croatia, it's the opening match, it's difficult, they are a good team.
"Forget the negative record and just think about the game and think about winning."
When asked if England can win the Euros, Mourinho added: "Can, 100%. Should? I think is a little bit too much is an extra pressure they don't need. Of course they can and have the potential.
"To say they should, you could say the same against France and Portugal. Should is too much. They should be positive and believe in themselves and go to every match and think they can win it."
Harry Kane and Euro 2020
A lot of England's hopes will be pinned on Spurs striker Kane, who Mourinho managed over the last 18 months.
He said: "He pays the price of his success and he pays the price of his numbers. People expect him to score goals all the time, which is what he does.
"I don't think he's very comfortable with it. I don't think he's even used to it. What I think is needed is what we tried at Tottenham is that he drops so much and gets in between the lines and is such a technical player that he can assist and can link the play.
"If you do that then you don't have a player who looks for that then you find someone who moves the ball and looks dangerous. In the end, he would naturally drop and leave his area. Great vision, passing and change of direction and explore the opposite full back.
"You cannot say only him can score goals.
"I did message yesterday. Now it's time to (stand back). Now it's time to close the door and leave them in their room with family and close friends.
"From my side, when a player is my boy, he's my boy even when we don't work together.
His England XI vs Croatia
Mourinho went on to name England XI he would start for the Three Lions' Euro 2020 opener against Croatia.
And there's no room for two of his former Manchester United players, Luke Shaw and Marcus Rashford.
"I think Gareth Southgate has great potential in his hands," he said.
"I would go with Dean Henderson in goal.
"I am a Henderson fan. When I was at United he was a kid, and I tell this story because it shows his nature.
"He came to my office asking for a loan, he went to Shrewsbury, but this kid said to me then, 'When I come back, I want to be number one'. We looked at each other because we had (David) De Gea and this kid never played one game, but he has had incredible development.
"The loans were all very successful. The way United organised them I think United did amazing for him and he did amazing for himself. He has this arrogance in him, this trust and belief which I think an England goalkeeper needs.
"Without Harry Maguire, I would go John Stones and Tyrone Mings as the centre-backs.
"Conor Coady is clearly a three defenders, defender. The kid from Brighton (Ben White) is a good player but he has just arrived so it doesn't make any sense to go directly in.
"Mings has good leadership qualities, he's aggressive, good personality. Stones is very fast to deal with space.
"Right-backs you can throw a coin but I would say Kyle Walker because you need pace, and a little bit of experience, an older guy, he closes the space inside maybe better than Reece James and Kieran Trippper.
"At left-back I would go with Chilwell. I wouldn't think twice. I'd go with Chilwell.
"I know that he plays normally with Chelsea in a five, but he is intelligent, he covers the space and the diagonals inside very well, he's dangerous in attack, he's good in the air and good on set pieces, he can defend well the possible long build up from Croatia on the first phase."
On Luke Shaw, he said: "He had a good season and clearly an evolution in terms of emotion, professionalism. But Chilwell has something more, especially with the ball, the way he thinks. He is very calm under pressure. I like Chilwell a lot.
"To play with Declan Rice, Jude Bellingham would be perfect but maybe this first game play Kalvin Phillips with Rice.
"I would have doubts here because Bellingham is the perfect player to play as the second midfielder with a guy like Rice.
"Harry Kane is untouchable, Jack Grealish is untouchable and the best position for Grealish is coming in from the left. For me, Marcus Rashford can only play there. When he plays on the right, he is completely lost.
"He is dynamic but on the right totally broken. He is very good on the left attacking spaces. But Grealish, for me, is tremendous and what he creates and his personality - 'give me the ball' - he reminds me a little bit of Luis Figo.
"I don't like making comparisons, especially with a Golden Ball winner, but Grealish reminds me of my Figo.
"Figo, I had him in different periods in my career but I had him in his last season as a player at Inter Milan and even in his last season he was like, 'give me the ball and I will resolve the problems for you'.
"The way Grealish gets the ball and attacks people, gets fouls, gets penalties, he's very powerful, I like him very much."
"So I would say Grealish on the left, Mason Mount as the number ten and because I like the wingers to come inside, I would play Phil Foden on the right.
"I would play Foden, Mount and Grealish, that is why I'm saying Phillips and Rice (behind them). Mount has an incredible football brain."
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