According to the BBC:
The contract will begin on 1 January and the Magpies say Noble will be eligible to play for them from the home fixture against Crawley on 6 January onwards.
In the meantime, Noble will train full-time with the club and feature in reserve matches.
It makes sense that he can't be registered before his contract starts which is why he can't play on 1st Jan 2018. The 6th Jan is of course the cup game now. As far as I know he's not played in the FA Cup this season so presumably is available.