The 36-year-old missed Tuesday night's 0-0 Barclays Premier League draw with Manchester United and has now been ruled out of Saturday's trip to Blackpool.
But manager Alan Pardew has admitted he may have to have an operation and a decision will be taken over the next couple of weeks.
Injury: Newcastle keeper Steve Harper may need a 'small operation' on his knee
Pardew said: 'Harps isn't right and we will have to review him over the next couple of weeks.
'He's not necessarily out for the season, but it could be that he needs a small operation just to sort his little problem out.
'It's not a major problem, but we will have a look at that and weigh that up over the next couple of weeks.'
Dutchman Tim Krul will once again deputise at Bloomfield Road with Ole Soderberg providing the back-up from the bench.
On-loan Ireland pays tribute to Toon Army after debut in United stalemate Newcastle 0 Manchester United 0: Red Devils fire blanks to give Arsenal hope NEWCASTLE FC