where the bath tub drain and p- trap I removed from my old tub was screwed onto the rest of the pipe going to the main drain (this is on the 2nd floor) there was some waste water that was pretty scuzzy. The tub/drain had not been used for over 6months, but I am wondering if the pitch of the pipes were correct should there be any water in this part of the pipe? I am wondering if I can just try and attach the new drain for the new tub and a new/clean p-trap connected where it was or if I might have a slope problem

If this doesn't make sense let me know and I will try to explain it better.