Southgate says injuries give England hopefuls chance to press case for Euro 2024
England, among the favourites to win the tournament in Germany, face Belgium at Wembley on Tuesday hampered by a long list of absentees including captain Harry Kane, Bukayo Saka, Kyle Walker and Harry Maguire.
Southgate's men are smarting after a 1-0 defeat to Brazil on Saturday -- their first loss since their World Cup quarter-final exit to France in December 2022.
They now face Belgium, fourth in the FIFA world rankings, in their final match before the June 7 deadline for the submission of squads, a week before the tournament starts.
Southgate has had to contend with a number of injuries to key players, both before he announced his squad for the Brazil and Belgium games and during the camp.
"It is an incredible number who are unavailable to us," said the England boss.
"I haven't experienced that before, but it means there are opportunities for other people. It is increasingly difficult at international level."
Southgate confirmed that Real Madrid midfielder Jude Bellingham could play the whole game, having recovered from the cramp which forced him off against Brazil.
He said Brentford forward Ivan Toney, who has just one England cap, would be involved against Belgium.
Toney earlier this month received his first England call-up since he served an eight-month ban for breaking Football Association betting rules, but he did not feature against Brazil.
Arsenal's Declan Rice will captain the side on his 50th appearance for England.
"He has great leadership and experience already at a younger age," said the England boss. "It's a great opportunity for younger players to show leadership.
"Different feel about the group, different dynamic. A lot of the players with the squad now are the future for the mid and long term."
Rice said he was "speechless" at being handed the chance to skipper the team.
"Fifty caps, it is a pinch me moment, it will be an honour," said the midfielder. "On my 50th cap, to walk out at Wembley in front of my family and friends, it is a great honour, I'm a bit speechless."