While we haven't seen him play CB at PL level , CB has been his position last season and this for the U23s. He made a few mistakes early on last season but improved throughout the season and his partnership with Galloway against Chelsea u21s in what may have been the last game of the season was very solid, though ironically I thought Galloway looked the better CB.
So in many ways , playing him at RB is a risk as it's not the position he's played most if his games in, so it revolves around the fact that RB is less exposed if he makes a mistake and the two CBs can cover him. The fact that he plays CB makes him ideal cover on the bench. The minutes gained at RB are invaluable to him and in gaining experience of the PL . The fact that he can play at RB and do well is a great testament to his ability he's developing into a very good player, but any suggestions that he should keep Coleman out should be considered nonsense. He's a year ahead of Jonjoe Kenny, and has better physicality and defensive qualities than Kenny, but Kenny is by far the better attacking RB and better at getting in penetrating crosses. It's a side that Holgate has never really had to develop like Kenny has ....... because he's played most of his games at CB.
This is where the Koeman regime needs to step up and be involved in Holgate's development . Unsworth is developing him into a pretty good CB at U23 level, if he's going to be seen as cover for RB then he needs further coaching and development at first team level , I.e by Koeman.