He was rushed to Mumbai's JJ Hospital due to chest pain.
"Salem was attacked by Mustafa Dossa with a sharp object. He was injured and doctors are treating him inside the jail. A police team is at the spot," Vasant Tajane, Senior Inspector at N M Joshi Marg police station, told PTI.
One accused was acquitted by the court.
The detailed role of the six convicts in the case, whose trial was separated from the main one, which concluded in 2007.
The Maharashtra government's decision to hand over all the cases against underworld don Chhota Rajan to the Central Bureau of Investigation has not gone down well with the Mumbai Police with several senior officials criticising the move stating that it has raised doubts over their ability.