The Red Devils have already strengthened their backline with the addition of French youngster Leny Yoro from Ligue 1 side Lille.
However, they are still keen to sign at least one more centre-back following the departures of Raphael Varane and Willy Kambwala, while there is also a possibility that Victor Lindelof could be moved on this summer.
The 20-time English champions have been heavily linked with a move for De Ligt, who is keen to reunite with his former Ajax boss Erik ten Hag.
It was recently reported that Man United were yet to meet Bayern's €50m (£42m) asking price, casting doubt as to whether the Red Devils will be able to strike a deal.
Man United receive De Ligt boost
However, the Red Devils are still believed to be holding talks with agent Rafaela Pimenta over moves for De Ligt and Noussair Mazraoui.
Bayern have now revealed that De Ligt is one of the absentees from their pre-season squad for the upcoming trip to South Korea.
De Ligt, Kingsley Coman, Dayot Upamecano and Harry Kane have all been left out, with the quartet still on their summer breaks following their involvement in Euro 2024.
Alphonso Davies has also been omitted from the travelling party after representing Canada in the Copa America in the USA.
As it stands, De Ligt will return from his summer break and link up with Vincent Kompany's squad on August 2.
Is Mazraoui in the squad?
In contrast to De Ligt, Mazraoui has been included in the pre-season trip which will see Bayern face Tottenham Hotspur in Seoul on Saturday.
However, according to journalist Florian Plettenberg, Mazraoui's involvement does not mean that a potential move to Man United is off.
The Red Devils remain in regular contact with the German side and have already reached a verbal agreement over a transfer.
Man United are now waiting to offload Aaron Wan-Bissaka, before they can push forward with their plans to sign Mazraoui this summer.