A man was grabbed by the ear and punched in the face as he waited in a car at a service station in Sutton.
The attack happened at around 7.45am yesterday (Thursday) at the Caldo Service Station on Worsley Brow.
Detectives say the 25-year-old victim was assaulted by a suspect, who sped into the service station on a motorbike, ridden by an unknown accomplice.
The victim was left with bleeding and bruising.
A 26-year-old man from Sutton has been arrested on suspicion of assault but has been bailed pending further enquiries.
The victim remains in hospital receiving treatment on the injury to his ear.
Anyone who witnessed the assault or has any information is encouraged to call St Helens CID on 0151 777 6811, call 101, or you can call Crimestoppers anonymously on 0800 555 111.