To be honest our biggest issue was scoring goals, our defence was pretty solid last season. The assumption is that by getting a DM we will solidify our midfield and protect our defence and allow more freedom to our more attacking midfielders (Pogba, Mikhi etc)
But TBH there's no reason why we couldn't approach that problem with a different solution. Put Herrera as the DM and put Renato as a box to box next to Pogba. Renato would give us that extra ability going forward, he can dribble and create space and also make late runs into the box. Signing him, if he can play to his potential, would surely go a long way to solving our midfield issues. I don't think getting a DM is an absolute must and most importantly of all we must solve our goalscoring issues