Mystery over how SA man became injured

A man with head injuries is in a critical condition in Adelaide but police don't yet know how he became so badly hurt.

The 30-year-old was dropped off by an unknown couple at a friend's address in Salisbury North on Wednesday night, and was then taken to hospital.

Police say the man hasn't yet been able to speak to them, as they try to piece together what happened between 5pm, when he was last seen in Davoren Park, and when he was dropped off.