"While she went to open L5 to close a step ladder, there was a gap between the ladder and the plane due to which she fell right through", a source in the airline said. "The matter is under investigation", an Air India spokesperson said.
Air India flight AI864 was scheduled to depart for Delhi at 7am.
Ms Lobo, 52, suffered a fracture and other injuries and has been taken to a hospital.
The incident came weeks after more than 30 passengers aboard an aircraft of Indian carrier Jet Airways had to receive treatment after pilots "forgot" to turn on a switch regulating cabin pressure. The flight eventually left after a 90-minute delay at 8:30 am.
The accident occurred at Mumbai's worldwide airport as the crew were getting ready to welcome passengers aboard the New Delhi-bound flight. A team of orthopaedic specialists are taking care of her, Nanavati Hospital chief Rajendra Patankar told reporters. She has been admitted to Nanavati Hospital.
"The woman crew member was closing the door for the pushback when she fell from the aircraft", the source said.
The plane was operated by Air India's low-priced wing, Air India Express.
An Air India Express 737 en route from the southern Indian city of Trichy to Dubai on Friday had to land in Mumbai three hours into its flight after Trichy Airport officials reported that the aircraft "might have" hit an airport perimeter wall prior to takeoff.
The tragedy happened when the co-pilot mistook a signal from ground staff and started the engine.