Mills knows exactly what it takes to perform on football's biggest stage. The former Leeds United and Manchester City full-back was part of Sven-Göran Eriksson's England squad at the 2002 World Cup, starting every match as England reached the quarter-finals before being eliminated by eventual winners Brazil.
Now working as one of English football's most respected pundits, Mills admits he has been left puzzled by several of Tuchel's squad decisions heading into the tournament.
"I was a little bit surprised by some of the omissions," Mills told Football Presse on behalf of Betway.
The former England international understands that tournament football is not simply about selecting the 26 most talented players available.
"You then need players that are happy to be second choice, are comfortable with that, are supportive of those that are starting, keep the standards very high in training, are not disruptive, and are happy to be the opposition in training sessions," he explained.
"It's a very difficult balance and it takes a certain type of character, which is why the likes of Jordan Henderson are involved in this, because they set the tone, they set the standards on the training ground."
Mills believes too much emphasis is often placed on assembling a collection of star names rather than creating a cohesive squad.
"We've always gone in the past saying, 'We just need to pick our best players.' We've seen time and time again that picking the best players does not make you a great team. That's a fact."
Pointing to examples from club football, Mills added: "Go to PSG with Neymar, (Kylian) Mbappe and (Lionel) Messi. That did not make the greatest team because you need to have that chemistry."
Two baffling omissions
Despite understanding some of Tuchel's thinking, Mills remains baffled by certain omissions, particularly those of Manchester United defender Harry Maguire and Chelsea midfielder Cole Palmer.
"Leaving Harry Maguire out, I do not get in the slightest," he said. "I think he's experienced. I think he's had a very good season. He's reliable. Would he be disruptive if not in the team? Absolutely not."
Mills also questioned England's defensive depth.
"I think defensively also we're quite light, especially in the full-back area, which would be a concern."
The exclusion of Palmer was another decision he struggled to understand.
"People say, 'Well, Cole Palmer's not 100 per cent fit.' Well, nor was Wayne Rooney, nor was Michael Owen, nor was David Beckham, and they all went to tournaments.
"You took them because they can produce moments of magic. Cole Palmer, for me, is in that pedigree.
"If you need somebody to come up with something in the last 20 minutes of a game, he can pick a pass, he can score a goal."
Group phase should be a breeze
Attention will also focus on Jude Bellingham and whether Tuchel starts the Real Madrid star against Croatia. Mills believes there are still unanswered questions.
"Jude Bellingham is clearly an incredible talent, but Thomas Tuchel still seems a little bit unsure," he said. "There's clearly something underlying that doesn't quite fit within that."
However, Mills insists England's fate will not be determined by what happens in the group stage.
"The first games are kind of irrelevant," he said. "It's the knockout stages that are going to be important for England."
Even so, Croatia will provide a significant early test. The experienced Balkan side have repeatedly troubled elite nations at major tournaments and remain one of the most tactically disciplined teams in international football.
Mills expects England to dominate possession against teams who sit deep throughout the competition and worries that Tuchel may lack enough players capable of controlling matches when pressure mounts.
"Sometimes it's not always about scoring goals. Sometimes it's about ball retention," he explained. "The best form of defence is keeping the ball because, if you've got it, they haven't. And if they haven't got it, they can't score."
As England begin their quest for World Cup glory against Croatia tonight, Mills believes Tuchel has taken a calculated gamble with his squad.
"Thomas Tuchel is either going to be a hero, or he's going to fall on his sword. Simple as that."