West Ham United youngster Declan Rice was named in the England squad on Wednesday for this month's European Championship qualifiers just weeks after pledging his allegiance despite having already played for Ireland.
His call-up comes in the same week that he was named Ireland's Young Player of the Year.
It comes after the defensive midfielder, who can also play in defence, switched global allegiance from the Republic of Ireland.
Rice, who can play as a centre back or in midfield, has been one of West Ham's most consistent players this season, earning rave reviews and comparisons with former club great Bobby Moore.
In February, Rice - who also played 20 times for the Republic at various youth levels after qualifying via his Cork grandparents - finally announced his decision to declare for England. When you're asking a player to transfer association, you've got to make sure they've got a future with you.
On the other hand defender Keane is now part of the squad and the England manager says former Republic youth worldwide Jack Grealish could be part of his plans down the line.
"We have had players eligible for us who have been with us at under-16s and other countries have given them senior caps very early", Southgate told BBC Radio 5 Live.
Southgate acknowledged Rice's switch of allegiance had been questioned by some but said it was a common problem.
And four months on from their last game, which saw England beat Croatia at Wembley to reach the UEFA Nations League semi-final, Southgate admits the situation is different for a lot of his players with their clubs and there has also been the emergence of other options.
"It's a complex landscape".
Rice is the only new face in the 25-man squad to which Burnley goalkeeper Tom Heaton his recalled alongside Jordan Pickford and Jack Butland.
After his heroics for England in the summer, this has been a very hard season for Everton goalkeeper Jordan Pickford.
"I think that he is ready to play for England and I think that we are just starting the development of him as a player but at this moment his performances is better than every holding midfielder [that England has]", the Chilean manager said. "He's only recently got back into the team at Southampton", Southgate added.