That's a pretty big field (for soccer) but I think the 400mm f/2.8L II is going to be your best option. I think you might find 600mm too long. You probably want to have a second body with a 70-200mm attached also.
The choice, 400mm or 600mm, depends on where you will be in relation to the athletes. If on the side lines, I think 400mm is enough. You can crop some and, if you have one, use a 1.4x extender on the 400mm. That will give you 560mm f/4. Not shabby and on a 1Dx should still AF very fast.