A man was assaulted close to Brierfield Community Centre, on Colne Road at around 4pm on January 9.
Two men have been arrested in connection with the incident and released on bail pending further enquires.
Officers investigating the incident would like to speak to a man they believe may have witnessed the assault.
A spokesperson for Lancashire Police said: "The potential witness can be seen boarding a bus near the library shortly after the incident and police believe he may be able to assist with their enquires."
Anyone with information is asked to contact 101 or email [email protected] quoting log number 1021 of January 9.
Alternatively, independent charity Crimestoppers can be contacted anonymously on 0800 555 111.