Best has shown no signs of recovering from an ankle injury after he returned from international duty with the Republic of Ireland last week.
He had just one training session and it raises the prospect of an operation.
Alan Pardew: “Leon Best is out for Saturday. The injury is such that we have got to get him seen on Monday by a specialist to see exactly what is the problem, because it just is not right.
“Are we worried he is going to be out for some time, maybe even the rest of the season? That is a possibility, yes. We will have to wait and see.
“I have no idea at this stage how long he could be out. When he has this injury before, he has an injection and it seemed to go away, but he has had another injection this time and it has not gone away. We might go down the line of having a second injection, we’ll wait to see what the specialist advises.
“It has nothing to do with Ireland’s training sessions in fairness to them. Should he have trained with Ireland? No, probably not. But I do no think it has had any impact.”