Alexander-Arnold started England's first two matches at the Euros - a win over Serbia and a draw with Denmark - in midfield. But Southgate opted to swich things around, with Conor Gallagher getting the nod against Slovenia and Kobbie Mainoo starting Sunday night's 2-1 win over Slovakia.
Throughout the tournament, Walker has been a mainstay on the right side of a back-four. But Neville was adamant at half-time that, with England struggling, Alexander-Arnold should have been given a shot in his preferred position and Walker should replace Kieran Trippier on the left.
"Trent Alexander-Arnold - it's illegal that lad isn't playing football in any game like this for England," Neville told ITV. "We cannot have Kyle Walker and Kieran Trippier right-back and left-back when we're on the ball most of the time. That kid is the best player we've ever seen at right-back or full-back in this country. He's got to be on the pitch."
England went in at the interval one down to Slovakia thanks to Ivan Schranz's third goal of the tournament. Despite the Three Lions' star-studded squad, Southgate has been unable to get a tune out of them since they landed in Germany.
Trent Alexander-Arnold was benched for England's last-16 battle with Slovakia (Image: Getty)
"It's now or never for Gareth," Neville continued before England eventually turned the game around. "He's got to interfere and rip up the script he's had all tournament. We've watched this now for three and a half games.
"He's tried to be really positive pre-match about this being the game where they turn it around but he's got to twist. We've got to see [Cole] Palmer, we've got to see Trent, we've got to see [Anthony] Gordon. He's got to shift the whole narrative around this game because that is not changing unless he does something right now."
We use your sign-up to provide content in ways you've consented to and to improve our understanding of you. This may include adverts from us and 3rd parties based on our understanding. You can unsubscribe at any time. Read our Privacy Policy
Former Republic of Ireland international Roy Keane, also on ITV punditry duty, agreed with his former team-mate Neville as the pair directed scathing criticism towards England's floundering side at the interval.
"He's got to make changes," insisted Keane. "No shots not target. Are they a team? Are they in this together? There's all these question marks. It doesn't look like it - there has to be changes."
Despite their protests, Southgate went into the second period without calling on any substitutes. Ivan Toney, Eberechi Eze and Cole Palmer were eventually called on as the Three Lions laboured and Slovakia stood firm.
And it wasn't until the 95th minute that Jude Bellingham bailed out his team-mates and struck with a spectacular overhead kick to force extra time. Less than a minute after that, Harry Kane struck his second of the tournament to make it 2-1.
Southgate will get another shot at proving his managerial prowess against Switzerland in the quarter-finals. Despite Sunday's narrow win, he will still have a point to prove and tricky selection calls to make.
