But the Liverpool midfielder is available to face Croatia, meaning Southgate is likely to keep faith with the same side which started the penalty shoot-out success over Colombia and the 2-0 quarter final victory.
England: Here's something no one expected: England, the most neurotic of global soccer teams, might actually have reason to be confident.
If Vrsaljko is ruled out then Dalic has two options, either bringing in specialist right-back Tin Jedvaj, who played against Iceland, or the more probable option of moving Domagoj Vida to right-back and bringing in the experienced Vedran Corluka to pair Dejan Lovren at centre back.
England head into the game as favourites with the bookies but most expect a tight and exhilarating encounter.
For every England match in Russia, Southgate has donned a waistcoat provided by Marks and Spencer, the official tailor to the England team.
"England have great players - they are a fantastic team", he added.
Southgate has been just the voice of reason England have needed to prevent anyone from getting too carried away.
"I wasn't a renowned fashion icon throughout my playing career [so] it's rather unusual to feel that way now", said Southgate, England's latest style guru.
Croatia topped their World Cup group in Russian Federation with three wins out of three games, scoring seven goals and conceding just one.
Fernando Hierro n'est plus le sélectionneur de l'Espagne — Officiel
L'Espagne a terminé première de son groupe B au Mondial russe, avec une victoire contre l'Iran et deux matchs nuls contre le Portugal en ouverture et le Maroc.
Strong, mobile, indefatigable and good in the air, Mandzukic looks to make life as hard as possible for the defence, stretching play to allow the likes of Modric the space they need to get into the danger zone around the box. They were completely out on their feet late against Russian Federation.
But Southgate thinks talk of that handing a physical advantage to England is being overplayed.
Not the threat of Croatia, Luka Modric's ability to carve open the best defences in the world or even Raheem Sterling's ongoing search for his first goal of the tournament.
Kieran Trippier has created 13 chances at the 2018 World Cup, more than any other defender at the tournament, and, if he can continue that form here, he may just provide Harry Kane with the bullets that fire England into their first World Cup final since 1966.
"But I like to challenge these strikers and to show to everyone that I'm one of the best defenders". It's important that we do our best on the pitch.
"We are concentrating on what's going on now". I felt something odd in my knee and after talking with the doctor on the sideline we decided I should come off.
Although Croatia are the most hard opponent England will have faced in the tournament, Southgate's words suggest he will continue with his attacking formation and personnel.
If England win on Wednesday, though, Southgate and his players will become more than mere fashion icons.
If it's level with 30 minutes to go, there's only one victor.