Either way, a lame-duck chairman
He has thereby lost all credibility. His best course would have been silence until forced to take a position on the Trump phenomenon. This he could have deferred for a long time. Now, If Trump is somehow nominated, he will have a chance of retaining the newly-meaningless job of titular head of a vastly diminished and irrelevant party.
But I wouldn't bet on it. If the Donald is the nominee, I'm fairly certain he'll want his own creature in that position.
If not, Priebus now would have to be replaced as head of a newly-purged but sadly also vastly diminished and irrelevant party facing the necessity of disassociating itself from Trump as completely as possible. This statement probably would put Priebus beyond the pale.
His is not a pleasant prospect in either case. Neither is that of the Republican party- or the nation. But whatever happens, Priebus's days as the chairman of a major American political party are numbered.