Gareth Southgate says England cannot afford to "stand still" in pursuit of progress after naming four uncapped players and surprisingly overlooking Kyle Walker for his latest squad.

Having finished third in the inaugural Nations League this summer, the Three Lions' attention returns to European Championship qualification with home matches against Bulgaria and Kosovo.

Southgate named an expanded 25-man squad for September's Group A double-header, with Alex Oxlade-Chamberlain and Harry Winks recalled as uncapped Mason Mount and James Maddison were brought into the squad.

Aston Villa defender Tyrone Mings was the surprise inclusion on Thursday, when World Cup semi-finalist Walker was the shock exclusion as Kieran Trippier returned and Aaron Wan-Bissaka received his maiden senior call-up.

"We tracked him very closely last year," Southgate said of the Manchester United right-back.

"We thought he had an outstanding season last season but we wanted him to stay with the Under-21 group.

"He's adapted very well and very quickly to a high-profile move. And playing in front of 70,000 fans, that brings intensity and pressure.

"We're looking at him and Trent (Alexander-Arnold) and that's the reason on this occasion that Kyle Walker isn't with us, we feel it's an opportunity in particular to have a look at two young players.

"We talked about the number of games we've got ahead of next summer.

"We think we need to see them quickly and we also wanted to reintegrate Kieran Trippier back into the group just so he knows that he's still on our radar, and he's obviously had a big – a huge – move to Atletico Madrid.

"Frankly, that kind of goes under the radar a little bit here and he's started the season really well.

"So, it is an area of the pitch where we have a lot of quality.

"Kyle has started the season really well, so I've explained to him the rationale for the thinking.

"It's not the case that he's not playing well for Manchester City – it's quite the opposite.

"So, yes, that is an area of the pitch with some really good, and four very different, options. They all have completely different strengths and we're looking forward to helping some of them develop."

Asked how Walker took the news, Southgate said: "I think he understands the thinking and I have a close dialogue with all of the players that are in the squad and particularly those that aren't in the squad this time.

"I've spoken with all of them to make sure they're very clear as to the reasoning why.

"In most of those cases they've missed a chunk of pre-season and the beginning of the start of the season, so that was pretty obvious for them. But we've got competition for places.

"I talked about only having eight matches ahead of a European Championship squad announcement – and we've got a depth of talent now because we've approached it slightly differently to look at younger players.

"It's increased the pool that we can pick from, it means that we lack some experience in a lot of positions but we've got to keep evolving as a squad and as a team.

"We've had a two-year period of progress and constant improvement I think.

"But we can't stand still and there are young players coming through that we might not see the best of in the next two years actually.

"But for the long term for England, we need to start integrating them because I think this team can constantly improve and evolve."

Southgate's selections have always had an accent on youth and a view on progress.

The England manager is a long-standing admirer of Mount and believes the Chelsea playmaker has been "one of the outstanding players" of the season, while Leicester's Maddison is another exciting talent.

More experienced figures like Fabian Delph, Eric Dier, Dele Alli and John Stones would have been pushing for inclusion was it not for injury, while the latter's absence opened the door to former non-league player Mings.

"I watched him last year and I wanted to see how he adapted to the Premier League and a different level of forward," said Southgate, who brought back Nick Pope for out-of-sorts Jack Butland.

"He's an interesting case because he's got into the game a bit later than some others, he hasn't played as much as a centre-back.

"You know, left-back at Ipswich, (he got a) bad injury at Bournemouth, so there's a lot of room for improvement with him.

"But, actually, I really like his leadership qualities, I think he's an outstanding athlete, very composed with the ball, so I'm looking forward to working with him because we see a lot of potential and room for growth given the number of games he's actually played."