"He was totally on board with us not playing him in the first game against Belgium". We have high motivation to perform and the chance to win a medal at a World Cup, which only one other English team has done, so there's a lot of motivation for us.
"In terms of mentality, it's obviously been a really hard couple of days for us", the England manager said.
The two teams have already met at this summer's tournament in the group stage, with Belgium beating England 1-0 on June 28.
The winger, 29, has scored twice in each of the last three major tournaments for his nation, and has become a crucial figure in Croatia's attacking setup alongside Juventus' Mandzukic, as was proven on Wednesday night when Perisic in particular was a constant thorn in England's side. And despite all 23 of his players training on Friday ahead of this match, Southgate was keeping his cards close to his chest with regards selection for Saturday.
"The honest thing is, it's not a game any team want to play in", said Southgate.
"The reality is that none of us know if that's as good as it gets", Southgate answered.
Southgate certainly has the nation's backing and provides a firm platform on which to build, unlike the teams of World Cup 1990 and Euro 96 when semi-final runs were followed by the exits of popular managers Sir Bobby Robson and Terry Venables.
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It is of course the Semi Finals that are going to be played next and we do now know which teams are playing each other. He's had a fantastic World Cup so far.
"They can now associate playing with England with enjoyment and fun and not being under siege".
The England coach was shattered by Wednesday's crushing semi-final defeat to Croatia in Moscow - and says he might never get over it.
It will take time to accurately reflect on England's "incredible experience" in terms of progress, individual performances and collective success, but their style as much as substance brings hope.
"We've all grown up watching England and to represent your country at a semi-final of a World Cup, there's no better feeling".
"There is a culture that exists now that any new player that comes in has to follow".
"It's enjoyable, we all love to play the game and fans love to support it".
Southgate added the perception surrounding England's national team has changed. We would be delighted if Modric won the MVP and the Golden Ball for being the best player at the tournament.
Hundreds of millions of people around the world will support us.