Not a Sprint customer but just guessing that the Sprint NFL app won't work because Verizon bought the exclusive rights to NFL mobile, and it is now a Verizon only app. Sorry, I can't recommend anything as I will be relying on NFL mobile this season.
Yes sprint has replaced that app with a app called Sprint football live. I used it to check out the pre game scores. When you 1st launch the app it says you gotta update. After the update it says you can only use the app on 3g.