It's not really about a DM per se, it's about having someone with defensive competence alongside De Jong.
Fred and Mctominay are tenacious and they cover ground, but they're just not effective enough in their defensive work - they're too easily beaten, too easily bypassed, lose too many duels, fail to track runners too often, position themselves poorly too often.
We don't necessarily need a player that sits in front of the defence and shields all game, but they need some maturity, awareness and effectiveness in their defensive work that Fred/Mctominay don't show.
Edit: they must also meet the bare minimum level of technical ability on the ball, which Fred/Mctominay fall short of.