If I was DT, I'd be most concerned about where I was going to get game time at this point in my career. Celtic don't have the best track record for fielding young players they buy (Ryan Christie and Lewis Morgan spring to mind), but I'm not sure Newcastle would be much better in that regard.
However, I'm not DT, and expect he'll make the decision that suits him best (as he should). In the meantime, I just hope we get at least another 6 months from him.