Ramakrishnan alias Ramki, a friend of Alagiri's son Durai Dayanidhi, was attacked by the assailants at Puthur area around midnight yesterday.
He had been admitted to a private hospital with serious injuries, police said.
The assailants were wearing helmet and could not be identified, they said, adding a search was on for them.
Ramki was one of the persons interrogated by the police in connection with the January 2013 murder of 'Pottu' Suresh, who was a personal assistant of Alagiri.