A man and woman were walking along Hartington Road when they were approached by a group of youths at around 8.55pm on Friday, March 18.
The group proceeded to seriously assault the man and punch the woman in the head.
The man, aged in his 50s, suffered a broken elbow and several cuts and bruises.
The woman, aged in her 40s, also suffered bruising.
Det Sgt Lisa Hough, of Preston CID, said: “This was a vicious and unprovoked attack on a couple who were simply walking to a friend’s house.
“The man in particular was left with serious injuries, and both he and his partner have been left extremely shaken.”
A man who was in the area came to the aid of the couple after watching the shocking incident unfold.
Police said they would now like to trace him and asked him, or asking anybody who knows who he may be, to get in touch.
"We would now like to speak to him to get his account of what occurred. If you think this is you, or know who he might be, please tell us,” Det Sgt Lisa Hough added.
“Similarly if you saw any of the incident, or have any other information about it, we would like to hear from you.”
Anybody with information is asked to get in touch on 101, quoting log 1420 of March 18.