Wayne Rooney feels England's 1-0 defeat to Iceland was evidence on how much of a 'big loss' not having Harry Maguire is to Gareth Southgate.
The Three Lions were booed off the pitch at Wembley in their final pre-tournament match against Iceland on Friday.
John Stones and Marc Guehi were the starting pair in defence, although the former had to be substituted at half time with an injury.
Rooney felt England's new-look defensive unit didn't look overly convincing and that Maguire's presence would have helped.
"Tonight the back line were a little bit disjointed at times, and that's through lack of experience," he said on Channel 4.
"That's something I'm sure Gareth and the coaches will be showing them, you have to get that right because we're so excited by the attacking players we've got, they have to be backed up by the back line and being aggressive.
"I think tonight actually shows the importance of Harry Maguire to this England team and what a big loss he is to this squad."
Maguire has been a staple of Southgate's England squads throughout his tenure in charge.
The 53-year-old admitted it was touch-and-go on whether Maguire was going to be selected, but the centre-back was ultimately not fit enough.
Southgate said: "I think we took nine [defenders] in the last couple of tournaments when I looked through, so I think we are the same [depth].
"That was part of the reason that, in the end, we couldn't take Harry Maguire because we would have had to have taken a 10th [defender] and that balance wouldn't be right.
"It would have left us short in other areas of the pitch.
"Harry has made some progress but it has been complicated and we definitely wouldn't have had him in the group stages.
"There were too many hurdles to get through without really being clear about where we might get to.
"Given that we haven't got a clean bill of health across the rest of the backline as well, the decision became that we needed cover and players that are fit and ready to go from the start.
"It was another really difficult call. You know how I feel about Harry Maguire, what he has done for England and what he has done for me as a manager."
Rooney still feels confident that England have enough options in attack to cause problems at the Euros.
But it's in defence where the Manchester United icon still has some concerns.
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He added: "It's a very exciting squad, they've certainly got the players and the firepower.
"I hope the backline is strong, that's my only concern.
"But I think we've got more than enough to go and take teams head on and if we do that we've got a great chance."
