He should certainly find a new trainer. Steward would be good, although I think you would have to question whether if Williams cant already fight from range, which is something he's bound to have tried to improve on in the past, whether he''ll ever be effective from range.
He can bounce back. Williams seemed pretty nonchalant about the KO in the post-fight interview. But I've been saying for two years that he leans in with his head and was going to get KTFO. He needs a new trainer, or needs to listen to his present trainer, who's probably been telling him all along that he should use his reach and height advantage and not to lean in with his head.