The 32-year-old has spent most of the last 10 years providing the back-up to the Irishman, whose legendary powers of recovery had limited his absences to a handful of games.
Manager Sam Allardyce stuck with Harper even when the younger man was fit for the visit of Wigan to St James' Park on September 1, and faces a tough decision ahead of Monday night's trip to Derby.
He said: "It's been incredibly hard. The four or five-year period I had out of the side was probably the hardest part of my life professionally.
"It was very tough and frustrating. When you don't get a game for such a long time, it's very much out of sight, out of mind, people tend to forget about you.
"You keep your head down and work hard, but it's tough. All I ever said was I want to play first team football here, and to play 25 games last year came at the right time for me because if it had not happened last year, then who knows what might have happened.
"That was important. It came at the right time for me, and the start of this season has been great and a big boost."