Manager Alan Pardew has used Campbell just twice and must decide whether to take up a one-year option on the 36-year-old's contract at the end of the season.
Campbell said: "I've enjoyed it here. It's been eventful and there have been ups and downs, but it's been good. I'm old enough to know you just have to get on with it as a player, and that is what we have done.
"At the moment, I'm fit. I've been okay for a couple of weeks and I'm happy to help out wherever and whenever I can. I'll play when I can and will support the team in everything they do."