The attack took place near Belha village in Lakhimpur Kheri district near Dudhwa forest and the man could be freed after a group of villagers came to his rescue.
The critically injured guard was rushed to a government hospital, from where he was referred to a higher medical facility in Lucknow.
Four people have been killed by crocodiles in the region in the past two months. On June 26, a farmer was killed when he was grazing cattle in Kheri's Singahi area.
On May 22, a man was killed in the Palia area, while two men were killed by crocodiles in neighbouring Pilibhit district.
According to the forest department sources, Eshwardin, a marginal farmer who also works as a watchman, was caught by a giant crocodile lying in wait for prey in the Sharda River canal near Nighasan range of Dudhwa forest.
The watchman shouted for help as the reptile caught him and began to drag him into the water.
Some six-seven farmers rushed to rescue him by pelting stones at the crocodile and beating it with sticks.
After a struggle of over 15 minutes, Eshwardin was rescued but his left leg was almost ripped off. By that time, he had lost consciousness and suffered excessive blood loss. He was then taken to hospital where his condition is said to be serious.