He was seen sitting on a bench in the park late in the afternoon on Saturday 29 October.
He was seen by a member of the public who called the police and said that he had an injury to his head but they had not seen what happened to cause the injury.
The injured man was with a group of other people at the time.
Now police want to speak to any of those people who may have seen what happened, and the injured man himself who had left the park before officers arrived.
One man possibly involved with the event was said to be in his early 20s with short blonde hair. He was wearing a blue puffer jacket and black trousers.
A woman is believed to have confronted the group and spent some time with the injured man. She was wearing a long, grey, duffel coat and she left the park with the injured man.
Any witnesses can contact PC 94 Hopkins or PC 113 Sweet on 222 222 or they can call Crimestoppers anonymously on 0800 555 111.