The project, currently titled More As This Story Develops, will focus on cable news anchor Will McCallister (Jeff Daniels) and his team.
Waterston has now signed up to play McCallister's boss, according to TV Line. The actor is best known for playing Jack McCoy on NBC's Law & Order from 1994 until the show's cancellation in 2010.
The actor also appeared in episodes of spinoff series Law & Order: SVU and Trial By Jury and was nominated for an Academy Award for his role in 1984 film The Killing Fields.
The Sorkin pilot will be helmed by Superbad director Greg Mottola and will also star Olivia Munn and Alison Pill. Emily Mortimer is also said to be in talks to play the female lead.
The West Wing creator Sorkin recently revealed that he is "loving the idea" of returning to television.
He explained: "I'm... taking the same combination of idealism and romanticism and realism that made government sexy on The West Wing and seeing if we can't do the same with the news and journalism, which are held in at least as much contempt."