Alexander-Arnold was one of the surprise picks in Southgate's recent squad, given his age and relative lack of experience, however the England boss is seemingly keen on trusting younger players, and he has come out in praise of the Liverpool right-back.
"Trent's the next best English full-back in the league," Southgate said, speaking to the press on Thursday. "His performances have been excellent."
The 19-year-old has stepped up to the plate for Liverpool, taking advantage of Nathaniel Clyne's absence to feature 33 times for Jurgen Klopp's side, including big performances in the Champions League against Manchester City and Roma.
"He's shown, in games of real pressure, the personality and the ability to cope with that," Southgate continued.
"He's had an extended run in a team where the expectation is high. He looks like he'll be playing in a Champions League final and is a player I know."
Southgate previously coached the England U21s, and knows Trent Alexander Arnold from the England youth set-up.
"I went to America with him two or three years ago with our U17s," he said. "He trained with us last month, no problem."
"There are some other younger players that aren't quite ready to be with us, but he's ticked all the criteria we feel we need."