I have heard about that Manchester United are thinking of replacing Anthony Martial with Tanguy Ndombele too. But actually, the positions of these players are completely different. Hence, Ndombele won't fill Martial's place in case this transfer happens. Maybe Manchester United are thinking of a big striker transfer next summer and want to just get rid of Martial at the earliest opportunity.
Maybe they are thinking of having a good defensive back. He is adaptable to lot of defensive positions so I think its a good choice given how United are performing defensively. They need a strong presence in defence and right now there is none. They could work out without Martial as he already is not finding much playing minutes after arrival of Ronaldo and to be honest it would be hard to compete with him for the position. Better offload from one department and add strength to other.
I think Manchester United have ready with new position will left by Paul Pogba, until right now not any new negotiation contract about extended or not for him, better if Pogba want to left Manchester United they need looking for new player can replace that position leaving, have more than six month to know which one player have good position with Pogba on midfielder.