Schalke has 8 wins in a row at home in Bundesliga, and by invoking some statistics rules we can say that the chances for another win are lower. However, this is a batter for first place, and I can't believe any statistics or this kind of things. The facts are simple, Bayern won't be 100% for this game, they players will have their mind on Old Trafford. Even if they would play in normal conditions against Schalke, it would be very difficult to win.
Robben is doubtful for this game, and probably will miss as he needs to be fit for the second leg in CL. I'm not sure if van Gaal will rest more players, I don't think so because this will cost the title in Germany. He needs at least to fight and try to win the title.
Probable teams:
Schalke: Neuer, Rafinha, Howedes, Bordon, Westermann, Schmitz, Kluge, Baumjohann, Rakitic, Kuranyi, Farfan
Bayern: Butt, Badstuber, Demichelis, van Buyten, Lahm, Ribery, van Bommel, Schweinsteiger, Muller, Gomez, Olic
You can also take an under 2.5 bet at 1.85 with Pinnacle, but for me this is the best bet: Schalke won't loose and if they will be focused they can win this game.