After what happened to other stars at United, Muller should think 100 times before agreeing to such a move 
It would be legendary for Müller to end his career in Bayern at the club where he started and spent his entire career. As a last resort, he could go to Saudi Arabia; going to United means a high probability of ending his career in an unpleasant way in an atmosphere of scandals and a collapsing club.
For United, this is also a strange transfer - another temporary option, another straw that the head coach is trying to grab onto.
IMO, going to Saudi Arabia means that Thomas Muller ends his career, it doesn't mean that going to Manchester United means that Thomas Muller's career ends, in fact in the European league the players can develop, indeed currently Manchester United's condition looks worse than last season when compared with Bayern Munich is of course different. Bayern Munich is still a strong club that always dominates the title in the Bundesliga and is a tough competitor in other European leagues.It would be legendary for Müller to end his career in Bayern at the club where he started and spent his entire career. As a last resort, he could go to Saudi Arabia; going to United means a high probability of ending his career in an unpleasant way in an atmosphere of scandals and a collapsing club.
For United, this is also a strange transfer - another temporary option, another straw that the head coach is trying to grab onto.
Thomas Murrel is an important player that Bayern Munich has with the goal assists he provides but considering his age it is no longer possible for him to continue playing so Bayern Munich intends to sell him to MU which is currently experiencing a decline and MU seem to needs Thomas Muller who has has a lot of experience in playing and providing goal passes to player who are relied on to scored goal.
Taking into account everything I know about Thomas Muller, I would absolutely rule out that he will ever go to Saudi Arabia to play there for any club ever. He his a serious Bavarian guy with deep roots in Germany and Europe and he is highly passionately running a horse breeding business with his wife and emphasized many times how important proximity to all of this is for him. I know that there is a pain threshold for everyone, but Thomas Muller is different than many others. He values what he has and he doesn't virtually add zeros to his bank account to then imagine how it would feel. Instead he stands by his word and loves the privileges he could obtain during life. I say no, he is not going to Saudi Arabia!
Manchester United could be an option for him and I think it was a consideration in the past if I am not wrong? But I think these times are over unless he gets some guarantee that he can play more than he can for Bayern. He doesn't like to be sitting on the bench although he accepts it, but he is also realistic. What are his chances at any top club to play most of the time?