I think that last season we were very lucky with injuries. The centre back pair of Tarkowski/Branthwaite hardly missed a game, Garner,DCL, Harrison, McNeill, Doucoure, Gana missed very few matches between them.
Others missed some games but most of our " first eleven" were generally able to play most games and at times it showed in performances where players were looking tired.
We may not be as lucky this season and signing O Brien is an attempt to deepen the squad. The fee is big for Everton in our current state but not a big fee by premiership standards.
We do still need at least one centre midfielder, ideally two, a full back that maybe could play left and right and if DCL leaves then another striker.
Even then we will have a squad that should be enough but still a long way off being a big squad.
Players like Holgate in particular and Michael Keane are players that really the club would like to move on but just may not be able to do so.
I would not be surprised either to see an older more experienced keeper brought in as extra cover.