Former England international Martin Keown thinks England should use a three-man defence if Harry Maguire fails to reach full fitness.
The Manchester United skipper, 28, faces a race to feature at the Euros after picking up an ankle ligament injury in May.
Southgate admitted Maguire, alongside Liverpool’s Jordan Henderson, has ‘work to do’ to make the starting eleven against Croatia on June 13.
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The manager has John Stones, Tyrone Mings and Conor Coady selected in his 26-man squad. Chelsea full-back Reece James can also operate on the right side of a back-three if needed.
Speaking about England’s defence, Keown said: “Maguire is playing in an area on the pitch where we are developing players but we are short of players.
“Both starters at centre-half, I only counted 12 caps between them before the Austria game so that was quite an inexperienced central defence.
“If Maguire doesn’t make it then with the players Gareth has picked he should shift back to a back-three and he could do that to great success.
“So it’s a big miss if he is not there and it is about changing the system to a back-three because he is that important.”
The former Arsenal man said Maguire is ‘key’ to Southgate’s team.
“I think we just find ourselves where we are developing new central defenders: Ben White and Ben Godfrey, Ezri Konsa at Aston Villa was no doubt close to a call-up,” he added.
“There has always been a production line of central defenders, we didn’t pick Eric Dier – I was a bit surprised he didn’t pick Dier even though he is fairly new to that position.
“Maguire is key – he came in at 16 to make his debut for Sheffield United and I always felt that he had all the star qualities you need.
“His presence in both penalty boxes, the way he passes the ball, his calmness, decision making, and I think he is quick enough for international football with the right people around him.
“That is probably why Southgate picked so many right-backs in the squad because either Kyle Walker or Reece James can play in a three.”
The Three Lions play Romania on Sunday in their final preparation match before the tournament starts.
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