A 29-year-old man from Spalding had his jaw broken in two places after an attack in the town on Wednesday.
The injured man was walking on London Road, away from the BP garage and towards town, when he was attacked from behind between midnight and 2am.
Police think the scene of attack was between Water Lane and Vine Street, but there are no descriptions of the attackers.
Anyone with information should call PC Ian Braid on 101, stating incident 188 of June 15.
You can also call Crimestoppers, anonymously, on 0800 555 111.