The last I heard is that Pochettino might leave PSG, in which case PSG want Zidane to take his place. But Zidane himself wants to lead the French national team. It's hard to predict how the puzzle will play out, but the likelihood of Zidane ending up at United is very slim. I think United's ability to sign a great manager will depend a lot on where they finish. If it is top 4, then it will be a plus.
That is the story in the news. These type of news not always come true but they come from inside stories that people have a chance to chat with the people in the right places and contact each other. I have seen too many transfers happening because of the news, like one player wanting to play for a team and tells that to a reporter "in confidence" and that reporter notifies the reporter that works for that club and club learns about it and starts to take interest in that player. This happens all the time, happens with managers as well.
If Pochettino fails to get the UCL this year which I am pretty sure that he will, then I know Zidane would want to manage PSG, it is at home, he is French, it's Paris, so what's better? That would be accepted by his wife too I am sure, they spent a lot of time in Madrid but France wouldn't be too bad for them.