Still, depending on how badly the Western Allies have been mauled in the north, and the fact that Antwerp has possibly been trashed in the fighting (oops: back on the Channel ports and Mulberries for logistics, which could really slow things down) it has the potential to pose a number of problems on the western front, which is what the OP is looking for...
Roosevelt is going to name a US general as commander to a British Army group. And Churchill, because he admires Roosevelt, and maybe if Roosevelt offers him a pat on the back/consolation somewhere in the Far Eastern theatre, is going to agree.
If Montgomery puts his foot in his mouth over Market Garden (maybe says something
really media-unfriendly about problems experienced by American troops during the operation) there could be a lot of pressure back in the USA to get rid of him, and since the operation was his and it's just failed...
Don't know enough about the US army politics and personnel to suggest a general to replace him who fits the requirements.
Edit: I'm theorising here, to try and work out a possibility other than the 'weather screws up D Day' suggested in the opening post.