Of course nothing is guaranteed, but Newcastle are big in Newcastle, Celtic are all over the world due to their Irish/Catholic connections.
Vastly bigger than Newcastle.
Infact I'd put them in the top 4 of British teams for International support behind Man U, Liverpool and Arsenal.
Currently Celtic would struggle in the Prem.
However, you're talking about a club with 60,000 crowds, huge International support and they would also be getting Prem money, much better sponsorship etc.
So after a few years they would probably turn into quite a force in the Prem with that type...