A 38-year-old needed hospital treatment for a fractured eye socket after an attack in Spalding over the weekend.
It happened near Tesco Express in Matmore Gate at about 5am on Sunday and the injured man was taken to Pilgrim Hospital, Boston, where he was kept in overnight until yesterday
Detectives are trying to tace a couple who may have vital information about what happened.
The couple are a white man, aged 20 to 25, about 6ft tall with short dark brown hair, and a female in her early twenties, large build, with blond hair tied back and wearing a red shirt over black trousers and light trainers.
Anyone with information should call DC Zoe Arnold at Spalding CID on 101, stating incident number 89 of May 25.
You can also call Crimestoppers, anonymously, on 0800 555 111.