The 40-year-old, from the South Bank area of Teesside, was discovered on the A66 in the early hours "in a distressed state" with wounds to his groin after the attack.
The road, from Greystones Roundabout to Church Lane in Middlesbrough, was shut earlier while searches of the area were carried out. It has now reopened.
A 22-year-old man was arrested on suspicion of assault and later bailed.
Cleveland Police said the injured man had been placed in an induced coma in hospital.
The force said in a statement: "Police can confirm that the man found this morning on the A66 had wounds to his groin area.
"Officers are continuing to investigate and are appealing for anyone with information to contact them."
A man has been found by the side of a road with his penis cut off.