The mishap took place Thursday when he was shooting for the "Shaktimaan" Pogo Chips ad film at the SRP Ground in Jogeshwari, near Mumbai.
"I was rehearsing when I got hurt. Everything was well organised for the particular shot. I was supposed to land from the sky. My body was attached with six wires and six people were holding me.
Due to some kind of miscommunication, three wires slipped and instead of a smooth landing, I landed with a thud on the ground and fractured my leg."
However, the accident has not dissuaded him from doing stunt scenes.
"I have to do it because it is not about heroes, it is about a super hero. But the kids should understand that even Shaktimaan can get injured and they should take good care of their body."
Khanna continued to shoot even after the accident and completed the shot.
"I told my producers 'let us do it'. We clumsily canned the shot and I think we can use it by using some tricks. But after that, I felt pain and could not walk. I called up my doctor who took an X-ray on the way. And when I reached Bombay Hospital, the doctors told me to use a wheelchair and then I realised the seriousness of the injury."
Khanna's foot is in a cast for the next four to six weeks and he is not to put any pressure on it. "It is causing a lot of inconvenience. I have to bear with it."
"This is a forced rest for me. Earlier when I used to visit my ailing friends or family members, I used to tell them, 'god wants you to rest, that is why he has given you this illness'. But this time, god has chosen me and the message is loud and clear - 'enough of work, it is rest time for you'."
This accident has affected the promotional tour of his serial on Disney Toon, "Aryaman", for which he was to travel to Pune, Lucknow and Ludhiana.
"I was feeling guilty about it, but the Disney people are really nice and said, 'You take care of your health and do not worry about the trips. We will cancel it and when you will be back on your foot, we will continue it."
Meanwhile, it is not all rest for Khanna as he plans to work on his book during this break.
"I am writing a book on health for children who are living in small towns and villages. I am using very simple language to make it easier for them to understand. I have done a lot of research for the book, but have not been able to work on it for last three-four months due to lack of time. I am going to utilise this time to complete the book."
Before working in "Shaktimaan" and "Aryaman", Khanna made it big on the small screen as the mythological character Bhishma in B.R. Chopra's TV series "Mahabharat".