Alexander-Arnold has been criticised by pundits for his defensive game as a midfielder.
But Southgate said ahead of tonight's Euros opener against Serbia: “Everybody’s more than entitled to opinions, so that’s our world, we have to accept that.
“Then we have to go and show what we can do.
“I’m really happy. I spoke to Trent about playing in this role over a year ago. It’s something he’s been really keen to do.
“He’s been really receptive to feedback and every bit of information. We know he can be a really special player. Equally, we know it’s not something he’s been doing for the last five years.
“There is an element of the unknown to it. But I think it’s an opportunity that’s worth grasping because he could be something incredibly special.”
He also said: “We think he can offer something we don’t have, he can give the team a different dimension and he’s been diligent in understanding the role. He has as good a passing range as anyone in world football really.
“He’s been invested in learning and understanding all aspects of the game in that role and I’ve enjoyed working with him on that project in the last 12 months. His mentality and attitude have been absolutely first class.”