I think these kinds of slumps are often caused by minor injuries that we don't know about. It depends on who it is though. If Mike Trout has a bad year, they are going to stay with him all year. If it's Javier Baez and you know he's extremely streaky anyway you stay with him. If it's someone with no track record, you remove him immediately. If it's Grossman who has been pretty good but not great and has reached an age where many players go into decline, it's a judgement call.