The man in his 40s was found on Common End Lane in Lepton shortly after 7:40am on Saturday.
He was taken to hospital where he was said to be in a critical condition.
Police are asking anyone who saw a man on his own carrying carrier bags to come forward.
Detective Inspector Lee Donnelly of Kirklees CID said: “Officers have been conducting a number of enquires locally and I am keeping an open mind as to what may have taken place.
“We are keen to understand his movements prior to being found this morning.
“I would appeal to anyone who lives locally who may have seen something, but not yet spoken to the police to come forward.
“Likewise, I’d ask those who may have been in the area on foot or in their vehicle overnight to think back.
“Did you see a lone man walking with carrier bags? If you did or have any other information that could help, please get in touch.
“Our enquiries are very much ongoing.”