I agree with most of the replies, our defence is an issue because of injuries and aging players, but mainly because of how much they are expected to do. We can't really play out from the back, so we go long and give them back possession and brace for the next attack.
Holgate has looked like he's a very good defender and capable of improving further.
Kenny has been good, but he has limited options. Have noticed a few times how well he tracks back and helps out centre backs when ball is on the opposite side. But when he doesn't have to expend so much energy in defence, you'll see him more capable going forward.
Funes Mori could still turn out to be a good defender, and intrigued to see how he'd get on with Holgate.
I'd like to see what Garbutt can do, he's maybe too late to become a top level player, but he always looked decent, just had limited chances behind Baines.
Also we do have quite a few young players who could find their way into the team. Browning seems to have be doing alright at Sunderland. We spent a decent amount on Lewis Gibson, not sure Galloway or Pennington, but you never know.
Jags, Williams, Baines to be gone before long, all past their best and Keane has really struggled. But if we can replace them with comparatively good, younger players, we'd be back in business before long.