I wouldnt be particularly surprised by a 0-5 start, even if I dont think that it will happen. The opponents next year are much tougher than the last 4 of last year, and the offense is still abysmal. Depending how the schedule works out, it could be very likely that the Browns start 0-5, as I doubt Harrison and JC will be able to carry the Browns all the way in games against teams who are better than (with the exception of Pittsburgh) 19th in rushing defense, and with better offenses.
However like others said, with the right offseason moves, our defense may be able to improve to a standard where we would still be in games come the 4th quarter, and can remain competitive with the ground game, as I dont see the passing game being a quick fix. I would still give Mangini the full season, although if the offense is still that bad I wouldnt mind MH stepping in that area at all.