The Red Devils have made their worst-ever start to a Premier League season this term and opted to part ways with Erik ten Hag after a 2-1 loss to West Ham United at the end of October.
With Ruben Amorim now at the helm, United are believed to be open to the idea of selling Casemiro in the coming months, even though the 32-year-old has had a strong start to this season following a difficult campaign last year.
The Brazilian midfielder is one of the highest earners at Old Trafford, and United are looking to cash in on him as he enters the final 18 months of his contract, particularly if they can secure a younger and more cost-effective replacement.
Man United face surprise contender for Goretzka signature
Man United have had a long-standing interest in Goretzka and have been rumoured to be lining up a move for him either in January or next summer when his contract with Bayern Munich is set to expire.
However, Goretzka's potential availability has caught the eye of several clubs, including a surprise contender from the Bundesliga in Union Berlin.
Die Eisernen's director, Horst Heldt, has confirmed that the club would be interested in signing the Bayern Munich star if he is open to a move while remaining in Germany.
In an interview with Bild, Heldt said: "It is known that his current situation isn't easy.. "He'll obviously think about his future. Any club can imagine having Leon Goretzka. We can also imagine that very, very well."
Heldt was seen speaking with the 29-year-old midfielder before Union Berlin's 3-0 defeat to the Bavarians earlier this month, and the two are well-acquainted, having worked together during their time at Schalke.
Goretzka among three Bayern players United are monitoring
While it remains to be seen if Man United formalise their interest in Goretzka, he is not the only Bayern player that the Red Devils are believed to be keeping tabs on.
Reports in recent weeks have indicated that the Red Devils are also considering moves for German international Leroy Sane as well as left-back Alphonso Davies.
Sane has yet to reach an agreement on a contract extension with the Bavarians, and as things stand, he is set to become a free agent next summer.
However, United are likely to have to fend off Arsenal to sign the 28-year-old winger, while Davies seems destined for a move to La Liga giants Real Madrid.