Basing the argument on current form, the decision can be argued. Not even Trent Alexander-Arnold's biggest fan would say his start to this campaign has been flawless.
Liverpool's right-back has had some difficulties, particularly in defeats to Manchester United and Napoli. There have been moments when the speed with which he has run back could be questioned, incidents when he has done things you had previously never seen him do.
Gareth Southgate, as England's head coach, has to select the players for matches he believes are in the best shape and have certainty in the calls he makes.