Structural preservation is required for making grammatical functions. Structurally, DE (得) has to be post-verbal to match its function. DE (得) in the sentence ‘*他吃飯得很慢’ is not post-verbal and is after a noun. So, verb needs to be copied before DE (得) to form a grammatical sentence ‘他吃飯吃得很慢’ [V-NP-V-DE-COMP].
