IMO Otamendi needs to be paired with someone who's content being the saftey valve, last line of defense. This is why Kompany's constant loss hurt us, because unless it's a set piece, he fills this role. That flat back 4 turns into more of a diamond as Otamendi attacks and VK drops.
Right now.. you have two "destroyer" type CB's and one (MDM) who doesn't have the pace or skill level anymore to play that sweeping spot.
We don't need a world-beater out there to fulfill this role, you just need someone who still has pace, and is content playing 100% defensively. Honestly that shouldn't be too hard to locate in this window. It's the scoring, attacking types of CB's that are in such high demand, and that's why we paid soooooo much for Mangala and Ota.