The officer was then approached by two men who struck him round the head, leaving him with fractures to his face and serious injuries to his shoulder and upper body, the force said.
The officer was discharged from hospital on Monday and is now recovering at home.
On Tuesday, BTP said that two 22-year-old men from Wolverhampton had been arrested on suspicion of grievous bodily harm and were being questioned in police custody.
Detective Inspector Gary Pinner said: “This is a pleasing development in our investigation into what was a shocking act of violence on an officer helping someone in need.”
He urged anyone who witnessed the incident and have not spoken to police to text 61016 or to call 0800 40 50 40 quoting reference 631 of 11/07/21.