Predicting a "difficult and intriguing match for both teams", the Liverpool centre-back dismissed the notion that Croatia are at a disadvantage because they have had to play 120 minutes twice in a week.
Phillip Schofield praised the team for what they had achieved and for uniting people.
"They'll be the best team we've played in terms of what they're capable of doing with the ball, and what their individuals are capable of, definitely", he said (via Sky Sports).
Southgate can reflect on two years in which he has taken England from the embarrassment of Euro 2016 to their first World Cup semi-final in 28 years.
Southgate said playing regularly in big matches was a key stage of development for his players and that the way they dropped away in the second half was perhaps a result of their inexperience.
"I've got a amusing feeling that the momentum is so good, and, you know, especially now that we're all going to turn up with special Gareth waistcoats, and that can only help can't it", said fan Lee Walkup, 50.
The Three Lions' failure to reach their first World Cup final in 52 years was greeted with desolation in the stadium in Moscow.
While England's top football players were on the pitch, its top golfers were sitting at home watching the game unfold.
Croatia coach Zlatko Dalic praised Luka Modric ahead of his team's World Cup semi-final against England, saying the midfielder was in career-best form.
"You've had an incredible #WorldCup, made history, and gave us fans something to believe in". I couldn't have asked for any more.
The game will kick off in Saint Petersburg on Saturday, before Croatia play France in the final in Moscow on Sunday.
Are you happy with England's World Cup campaign? "We will see after this practice session", he said. "I think it's going to be incredibly loud in any bar we end up going to for the match".
Gareth Southgate, his entire squad and support staff, looked in a daze as they stood just 20 yards from the banks of England supporters who had flooded into Russian Federation fuelled by hope and the desire to witness history.
"This is fantastic. Two players played with half a leg, but it didn't show".