Herrera had his best season with us as a defensive midfielder. We may well sign a DM but on last seasons evidence herrera is outstanding there.
I don't think so my friend.
He had his best season with us as the midfield link between the defensive midfielder (Carrick) and the front line players. He also had a great season as a man marker in the middle of the pack (vs Eden Hazard at Old Trafford quickly comes to mind).
Herrera is actually weak as a defensive midfielder
compared to what he is able to do when he is given license to roam forward. He is clearly a forward thinking midfielder who is
relatively poor defensively even though he makes impressive contributions in terms of putting in defensive shifts (e.g tracking back, tackling and intercepting) in midfield.
Why would any manager want to put Herrera right in front of the back four and ask him to sit there as a DM? That would be a waste. I remember the games where he was chosen to replace Carrick last season. He never stood out in those games. Yes, he did enough but he was never exceptional. What we need is a Carrick type of replacement who can dovetail between being a defensive/holding midfielder and also a central midfielder (like Carrick does so well) who can dictate play. Herrera is a work horse with a football brain. He can be brilliant as the midfielder who runs around and links up the back and the front. He can offer defensive support to allow a Pogba to be creative and attack minded. He can pick up the ball from a DM and take it to the attackers. He cannot be the player you saddle with the responsibility of shielding the back four throughout the season.
Think about it. Look back at last season. What stands out for Herrera? That brilliant tracking of Hazard. That through pass to Rashford that killed Chelsea. That lunge forward against Chelsea to score his own goal. That incredible through ball to Mikhi against Tottenham that led to a goal. That lob to Zlatan that led to the decisive goal against Southampton in the EPL final. That pass to Zlatan against Sunderland that led to a goal. etc.
For Herrera, it is always when he is able to serve as the link between the backline/ defensive midfield and the attacking players. That's when his star shines brightest.