Priority is 100% central midfielder.
Someone who can carry the ball and cause problems with it. Basically a Dembele replacement (Seri or NDombele preferably).
A new left back. Someone who's going to take on his man and beat them. Quick and gets themselves into good positions. From the media rumours of us scouting numerous left backs, this does seem to be a priority which is good.
Unless Aurier pulls a finger out, a new right back too. Though I think with better coaching, Poch's fitness regime and a bit of confidence and freedom to be allowed to run forward and beat a man, Aurier would be pretty damn good. (In a wing back, 3 at the back role preferably)
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Anything else would be a bonus. I think we're well set in the attacking sense. Eriksen, Dele, Kane, Lucas, Son and Lamela is fine. Though if we were to bring in Malcom then even better. I think he offers something different to all of those.
I also wouldn't mind us looking at bringing in a promising central defender in Lenglet or de Ligt - so if Toby does get sold, replacing him with one of these would be best. Unless of course Poch would prefer a bit more experience.