That is an option, and if you can go without is, the 400 5.6L is an excellent lens that is very light and small, and gives an extra 100mm of reach. I love mine. Way cheaper too.
Are you a professional sports photographer? Otherwise, I think if you don't know what to do with the lens, you probably made a mistake spending that much money on it.
No but i have been doing indoor high school sports and getting into wildlife. Thought I would try is it aince my 70-200 makes me crop too much. I bought it for that purpose but just trying to justify if its worth it or if I should get the 300mm f4 which is cheaper.