The current back four quartet of Nicky Hunt, Fernando Hierro, Radhi Jaidi and Bruno N'Gotty have conceded six goals in the last two games and Allardyce says that is a record which needs to change, starting with the visit of Kevin Keegan's side on Saturday.
"It is extremely worrying that, despite the work we have put in to get it right, all I have been talking about for the last two weeks is the state of our defending.
"One wonders where to turn to next and the first option is to change the side, particularly the back four. This one has shipped six goals in two games and can't stay together.
"This is relegation stuff and we haven't been in this position for a long time. I knew November and December would be tough but not this tough."