Walker, James and Trippier also made the final cut, leaving Southgate with a wealth of options in the position and as many right-backs as established centre-halves.
Southgate defended the decision by stressing that all four players are versatile and capable of operating in different positions.
Walker has previously played as a right-sided centre-back for England, while James has recently taken up that role at club level with Chelsea.
Trippier is capable of playing left-back, while Alexander-Arnold has long been touted as a potential midfielder having played there in his youth.
Southgate believes that the versatility of the four players and their ability as footballers justifies their selection.
"We went through this last week. Yes, we have four players who sometimes play at right back for their clubs. Kieran is here, he's been playing at wing back as well as right back," he said.
"Reece James has played the right of a three or right wing back. Trent plays at right back. And Kyle Walker for us at centre half as a three and at left back during a match for us against Denmark.
"Reece James was in midfield against Aston Villa. Basically they are in our best 26 footballers and that's why they are in the squad. If I could have picked a fifth or sixth right back I would have done."
Southgate added: "We're here to play nine games. We know we already have some injury doubts in the group. I'm sure we're going to face other difficulties over the next five weeks.
"So we need adaptability, good players, good professionals, tactically savvy players. We're fortunate to have old and young players who fit those categories."
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