"That is their doctor."
Reversing the subject with the object, we have 他們的醫生是那個, where 個 must be included to identify the type of object.
This type of sentence is not commonly said or written. It is most common to use the main one in the conversation.
Also, 那位 would likely be used rather than 那个 in order to be polite, but this still sounds awkward compared to the original word order.
Did the same, native speaker taught me that since you don't own a person, "de" is optional.