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.
but if we talk about strikers at Manchester United they don't really need to worry even if this happens, considering that until now there are still several names that can indeed be used as options.
other than that I think this exchange will be good for MU considering Tanguy Ndombele is one of the players who can indeed be used as a good defensive midfielder. even though they now have several defensive midfielders, the advantage of this ndombele when viewed from his game he is very creative and can be an option in the starting line-up.
I guess that they would also need to reconsider what to do with Ronaldo if they lose Martial and looking for a striker as well. Martial could play "around" Ronaldo while Ronaldo plays ahead, which means that the player they need to get should be a good scorer but not a pure striker, not like a poacher level player. Martial is perfect for that, greenwood is perfect for it as well, even Rashford works, but Ndombele? That doesn't work at all because he is a midfield, defensive midfield level of player and he is at the back.
I assumed that this was a discussion they had because of Pogba. If Pogba leaves but they get Ndombele to replace him, that could actually work quite well and that is what I assumed would have happened. We do not know if Pogba would leave or not, we still have no idea about that, but if he does, Ndombele could be the 100% perfect replacement.