Good at what he does but a bit limited on the ball I think.
Not necessary. What matter is what the midfield as a whole provides for the entire side. That being, defending, attacking, energy, and controlling the game through passing. With Pogba and Ndidi, the only thing missing would be someone who can control the tempo of the game. Herrera, Matic, Fred, nor McTominay can provide that.
Pogba carries the attacking responsibility of the midfield. What he lacks is the constant energy, the desire to control a game through his passing, and he's not someone who wants to defend. What Juve had was Vidal and Pirlo. Vidal provided the energy to attack and defend, while Pirlo provided the constant passing presence and helped defend by positioning himself well (as he didn't have legs). The midfield as a whole therefore provided everything.
Right now we have Herrera and Matic. Herrera does the Vidal role, albeit in a poor man's version. And Matic is supposed to be the Pirlo, but obviously does not have the passing ability of Pirlo, so everybody knows he's not going to provide that. As a result, United's midfield does not provide all it can to the side. We miss two key elements, passing and defending. If we had Carrick from 2013 substituted in for Matic, we would be golden.
If we had Modric instead of Carrick, we would no longer need someone like Kante. Why? Because Modric is more mobile than Carrick and so we wouldn't need some energetic midfielder to compensate for Pogba and Matic/Pirlo/older Carrick. We could have someone like that, but it wouldn't be as needed as it is with that 2nd midfielder being as slow.
Ultimately what matters is the midfield as a whole providing everything, no matter the difference in the puzzle pieces. If we didn't have Pogba, we would now need more midfielders being a little better than Ndidi at helping out the attack. I'm assuming we have Pogba, so I do think Ndidi would fit in. Ndidi lessens Pogba's defensive responsiblity, and Pogba lessens Ndidi's attacking responsibility.