David head on his body????!!

Perhaps Shaw stitched him back together, one optical fibre at a time -- out of the goodness of her god-fearing heart. After all, she has time to kill -- ten years in fact!

Alternatively, perhaps David can self-heal thanks to nano-bots living inside him, as long as his separate parts can be brought into touching proximity (again Shaw's help needed).

Finally, an outlandish theory of David applying 'black goo' to the separation cut (once head is replaced), which then works its magic a different way than with biological material, and simply heals his wound, or better yet, makes him a real boy!

Yes its a Plot Hole if you would but then with David to guide her we have to accept she can perform the task.

We have to accept Davids head was back on prior to seeing the Juggernaught leave at the end of Prometheus.

Impossible THE engineering are approximately 100.000 years Ahead in technology they cant put David head back. THE engineering are a druid kind of People searching through technology THE door to their existance

although seeing from Prometheus and THE hyper sleeping chambers of them I doubt Ridley scott made a wise decision to film THE hyper sleeping chamber scène because THE plastic tubes attachéd to THE engineer looked totally fake! And then THE scène were hè rips off David's head was THE story killer to alien covenant