Instead of a cast get them to strap it.
Almost everyone gets a cast put on because it is easier and causes less pain.
But you can asked to get it strapped. The advantage of this is that you use it more and more which stimulates it to heal and also means you don't loose as much muscle bulk.
Dissadvantage is that it hurts like hell for the first week.
Jockeys get this done quiet alot. My friends dad got it done to him and he didn't suffer after the six weeks, no cramps bit gimpy but soon got back to normal.
Its worth asking if its an option for your injury.
MJ