Sad to see Ghost-Sensai gone, but I have a feeling the author won't just trash such a good plot device permanently. Also, this is a good thing for the kid's character development. He's leaned far too much on the backing of his master. He will only grow truly strong by facing future challenges and hardships relying on his own strength, which he needs to continue cultivating.
For once, I don't have any criticism of the author.