"He can never replace Salman. He is a great actor and a very big star. Varun is like his younger brother and that's why he tweeted also. Salman loves him," David, who worked with the actor in "Partner", told PTI.
Johar last month confirmed the new cast of his film. "Shuddhi" first went to Hrithik Roshan, Kareena Kapoor and Salman before Varun and Alia Bhatt came on board. Salman tweeted his approval and support for the "Badlapur" star after the casting news became public.
"I just told him that it's a tough world and you have to work hard. All the credit goes to his sincerity and honesty. He gives his full effort to the films he does."
The "Hero No 1" director is currently enjoying a small break from Bollywood and would start writing his next project by August 2016.
His last directorial venture was quirky comedy "Chashme Buddoor".