He will cut him to pieces: Harsh physical punishment like this was known in the ancient world. It is not clear whether Jesus was describing punishment in this life or final judgment.
assign him a place with the unbelievers: This could mean treating him like an unbeliever in this life, or condemning him with unbelievers in the final judgment. The meaning depends on whether Jesus was speaking about discipline for an unfaithful believer or condemnation of an unbeliever. In either case, the warning is clear: an unfaithful servant will face severe judgment.