This is a good time to use the U1 memory setting on your mode dial. The idea is to make all of your settings adjustments now, then any time you need them you just dial back to U1 and the settings are all there, ready and waiting.
These are just my thoughts: I don't know what the minimum shutter speed is to capture action in soccer or motocross, or what length lens you use, but go into Manual mode and set your minimum shutter speed necessary to capture action at your focal length (1/500th? 1/2000th?) and your minimum aperture to get a decent depth of field (5.6? 8?). Change your ISO Sensitivity Settings to Auto in the shooting menu. That way, the camera will adjust the "exposure" sensitivity to match your shutter and aperture setting. Set your autofocus mode to AS-C with anywhere from 9 points to 51 points (or even 3D?), depending on how much of your frame the action fills. Set your camera to record in RAW + Jpeg fine. Now go into your Menu Settings and save all of these settings to U1 (Menu > Setup Menu > Save User Settings > U1). These are all now saved to U1, and no matter what you change for other shots, going back to U1 will restore these same settings.
At the actual shoot, depending on how fast you have to shoot, change your shooting dial (which is under the left Mode dial) to either Continuous Low (with time to autofocus between shots) or Continuous High (which locks in the focus on the first shot then fires a burst at that setting). Set the camera to U1 and give it a go.
Or you could just be like J-see and shoot everything in manual mode at ISO 100 and shutter speed whatever you want and aperture whatever you want, and then use post processing to adjust the ISO up to where you want.
It's technically sounder than it sounds now, but you won't see any good shots on your camera's LCD screen until you adjust them all on your PC later.
Just out of curiosity, have you tried shooting in Program mode? I really want to know if the camera computer will make all of the correct adjustments and figure it out better than we can. That gives me a good idea for some weekend experiments. The D750 also has a sports scene mode that defaults to higher shutter speeds to capture action.