TBH I have noticed spaces behind Herrera and Pogba when they play together alone last season but either our defense was just too good or the opponent was dumb to not use it.
Herrera isn't a DMF by nature. He's a b2b player so he rushes upfront too often to try attacking and this leaves space behind him and Pogba so when we lose the ball both Pogba and Herrera backtrack too much to cover this space leading to the whole team getting drawn back so when we cut a ball we find a difficulty to build the play. The other solution is to ignore this space behind Herrera and depend on Herrera backtracking only to cover it but the space was still present and a good enough opponent would have benefited from it.
Matic is better in covering this space and he knows exactly when to stay back and when to press upfront because this is his position by nature, Herrera not as he's a b2b by nature not a pure DMF.