The way I interpret it is more like: "I must have broken my leg". For instance, a reasonably common sentence is: "hij zal het wel gedaan hebben", meaning, "I guess he's the one who did it", or "I'm confident he did it". Or, if you're recounting an occurrence to the police or judge and you're not quite sure what happened, but expressing what you think is the most likely course of events.
In English, that word order implies someone else will be doing the breaking! "To have something done" means someone else will be doing it TO you - not the same as: "To have done something". E.g. "I will have my hair cut" (I'm going to the hairdresser), not: "I will have cut my hair" (I'm going to do it myself). "I will have my leg broken" sounds like you upset a criminal gang. ;)