The key problem of this partnership is that both players are slow. Therefore, when we have more possession, they are far away from attacker to be able to control runs. Cause of that, rival forwards will more often be able to easily get long balls and include midfielders in the attack. It could be prevented with excellent DM who would control space around rival attacker, but we don't have one.
Another problem is that Lindelof is weak in duels and once again needs help from DM or FB to be able to control long balls. If FB up the field, DM is crucial.
So, the key signing is DM. Someone who is able to control everything, and open space for Pogba and McTominay/Fred/whoever to be creative upfront and free to make mistake.