Just wasn't selected I guess; isn't injured anyway. Kenny & Alexander-Arnold are the RB's whilst they chose Fry, Gomez, Tomori and Worrall (Forest) as the CB's.
Can't really argue with that as he's not as good at RB as those two and hasn't got much experience this season at CB.
Full squad for reference - very strong:
Goalkeepers: Angus Gunn (Norwich City, on loan from Manchester City), Dean Henderson (Shrewsbury Town, on loan from Manchester United), Freddie Woodman (Newcastle United)
Defenders: Trent Alexander-Arnold (Liverpool), Ben Chilwell (Leicester City), Dael Fry (Middlesbrough), Joseph Gomez (Liverpool), Jonjoe Kenny (Everton), Fikayo Tomori (Hull City, on loan from Chelsea), Kyle Walker-Peters (Tottenham Hotspur), Joseph Worrall (Nottingham Forest)
Midfielders: Lewis Cook (AFC Bournemouth), Tom Davies (Everton), Kieran Dowell (Nottingham Forest, on loan from Everton), Ruben Loftus-Cheek (Crystal Palace, on loan from Chelsea), Ainsley Maitland-Niles (Arsenal), Josh Onomah (Aston Villa, on loan from Tottenham Hotspur), Harry Winks (Tottenham Hotspur)
Forwards: Tammy Abraham (Swansea City, on loan from Chelsea), Dominic Calvert-Lewin (Everton), Demarai Gray (Leicester City), Ademola Lookman (Everton), Sheyi Ojo (Fulham, on loan from Liverpool), Dominic Solanke (Liverpool)