"We have got 51 points already and in the whole of last season we only got 54, but that does not mean we are certainties," he said.
"What I don't want is to lose a massive lead like Kenny Dalglish with Blackburn and Wolves last season. They each had promotion wrapped up and then blew up and I don't want that to happen to us.
"That is why we need players like Ferdinand."
Spurs boss Glenn Hoddle has admitted he will allow the 35-year-old to leave White Hart Lane if he gets an acceptable offer.