Keeper
I think the most important and transformative signing for the first XI is a ball-playing keeper for several tactical reasons.
After this comes the rest imo.
Central midfield
I think our depth at CM is worrying and that's where I'd put money next, 2 injuries to our 3 starters and we'd be struggling to control the middle and it throws our whole formation/game plan out the window.
Fullbacks
We need a long term plan for what's happening in both full back positions. Currently, Livramento is twice the player at LB as his at RB. Hall is just as good in a different way. What's the plan for RB? We play Livramento there until he comes good? If this is the case we're OK squad-wise for next season for the two positions presuming we're keeping Trippier and Krafth. If Howe see Livramento as starting LB and keeping Hall out, I think I'd really like to see Lewis Hall as the third man in the middle with Bruno and Tonali as we progress to a more possession-orientated, front-foot team. Probably Football Manager thinking but he's played there before in his youth, he's got great close control, passing and doesn't get flustered and hounded off the ball. Could he be the deep 6 we've been looking for - letting Tonali and Bruno be more dangerous near the opponent's box? Joelinton would be a great option when we're under the cosh or against strictly better players as his strength, ball carrying and engine is phenomenal. Not so good vs teams who sit back though, and I think this is where someone like Hall would be a net gain.
Right wing
Murphy is fantastic when he gets time on the ball, but in games where he doesn't, he's really only contributing defensive effort. We need someone who's good on the ball and can work hard defensively. This will be the hardest position to buy for I think. I have no suggestions, I'll see who turns up. It's unfortunate that it seems clear Gordon isn't going to be playing there, I think it plays to a lot of his strengths and would solve a £50m problem for nothing.
Edit : Striker
We need a body who can take minutes off Isak, anyone cheap, young or old but experienced and can do a job even if he won't uproot trees.