For me;
Right back -- Kenny and Holgate are both fine. Holgate had a lot of games for us and England u21s here and Kenny was arguably player of the u20 world cup, has had loan spells and people who watch the u23s rate him very highly.
Left back -- Baines isn't the player he once was and there's no way he can play across 4 competitions. I think Koeman is looking at Connolly to see how he does in preseason and decide if he stays as rotation or goes on loan and we sign someone to take over from Baines .
Centre back -- Michael Keane is the starter and other than him we have two fast fading centre backs in Williams and Jags who are fine to rotate in/out but not to play consistently. Pennington needs a loan with a view to a sale.
I'd be hopeful we splash big on a partner for Keane. The only reason I could think not to do so would be if Koeman just wants to get a year more from Jags and Williams then make a superstar centre back his priority signing next summer. The same with Baines, perhaps he wants to give the young England u20 players a go?
Holgate plays like a CB playing RB, hes not fine at all, the lads only young, but hes not a good RB, hes semi decent, his future, if he bulks out will be at CB, if not, it will be in League Two.
Thankfully Cuco will save our right side.