Typically, you fix the spine/core of your team and then the limbs - the limbs don't matter so much if the torso can't do its numerous jobs correctly.
Keeper is a role we take for granted, but we've seen how important they are by way of the CL semis - we've won big trophies with great keepers and lost out on trophies with awful ones - keepers aren't a problem until they are... silent partner, like not knowing you have hypertension. Or something.
CB's, midfield (DM, CM, AM) and then forward.
I'm aware of our problems elsewhere, but you're absolutely fecked if you have exploits through your centre. You can mitigate for everything else, or fix them in stages. The centre doesn't allow for that as it is a pure co-dependency; one shit CM drags everything around him down. One bad keeper can destabilise an entire backline; one bad #9 completely bottlenecks the attack, etc. etc. fixing the central core is always a matter of the utmost urgency, which is why it's panic stations when it goes bad. Like, real panic stations, not the bemoaning stuff of a weak flank.