That's where the problems lie. Then, it's a case of what Koeman and Walsh are doing to rectify it. And they clearly need more time on that score.
I will say that Koeman needs to be bolder with selecting Davies and Holgate, shipping out Jags and others, and letting Barry out to pasture. In addition, he has thus so far failed to set us up in a way that maximises the advantage of having Lukaku in the team - something we very sadly have not been able to capitalise on for 2 1/2 years. He has also recognised the limitations of his honesty and forthrightness, whilst that was refreshing to begin with, calling out Barkley in public amongst other things has proven to be not the best policy.