The actor will be playing an army captain in the Hindi remake of Thuppakki.
A source told Bollywood Hungama: "Director Murugadoss was keen on starting [production] in the Christmas week. And Akshay was happy to oblige.
"The only problem is, he has two weeks to lose at least 5kg, and some more later to play an army captain at least 12 years junior to Akshay's biological age.
"Akshay is one of the fittest heroes in India. But to play the soldier we want him slightly fitter. His shoulders should become more streamlined, his frame should be slimmer.
"Akshay has the physique for the role. He just needs to lose a little more weight and become slightly sharper around the edges."
The posters for Kumar's latest film Khiladi 786 have been banned in Pakistan by censors.
However, Kumar has reportedly planned a sequel to OMG Oh My God despite the film facing similar controversy in the UAE.