Officers were called to the park at about 5pm on Saturday.
The man in his 40s was taken to hospital. Police have not released any further details of his injuries.
A spokesman for Lincolnshire Police said: “The park has been closed to the public while our investigations continue.
“We will open the park as soon as our investigations conclude.”
No arrests have been made as of Monday morning.
Anyone who can help with any information is asked to contact police on 101, quoting incident 248 of 28th