Dier has been a regular since Southgate was appointed as England manager and shone for him at the 2018 World Cup in Russia.
Southgate instead opted to go for the likes of uncapped pair Ben Godfrey and Ben White and he has admitted it wasn’t easy dropping Dier.
Asked if it was a difficult conversation with Dier, Southgate said: “Yes, definitely.
“He is a man who I have tremendous respect for and he has been, as you’ve rightly identified, a key player in some of our biggest nights, some of our biggest moments.
“Unfortunately I just don’t think his season has been strong enough to be in the squad.
“And that’s a difficult call because I know how much he brings to the whole group, and when you’re away at a tournament, those team players, those people that put the team first, are critical.
“So that wasn’t an easy conversation. But he’s an outstanding professional and he took the news in a really professional way.”