Sources told ESPN senior NFL analyst Chris Mortensen that the Patriots will use a franchise tag on Cassel that will give the team two options: trade him if all goes well with Tom Brady's rehabilitation from a knee injury or keep him because all is not well with Brady.
The Patriots' intention to franchise Cassel was first reported by the National Football Post.
"It's been talked about quite a bit [publicly]," Patriots spokesman Stacey James said Monday. "We won't be commenting on it until something happens. And if we don't use [the tag], we probably won't comment."
David Dunn, Cassel's agent, did not return telephone calls seeking comment.
That would mean the Patriots would have about $29 million in salary-cap space tied up in two quarterbacks, with Brady earning almost $15 million and Cassel guaranteed over $14 million.
However, the cap jumps to $123 million per team, which gives the Patriots $94 million to manage the rest of their roster.
"That's a smart move. You don't want to go into a season with uncertainty at that position," Patriots safety Rodney Harrison told ESPN's Rachel Nichols. "We figure Brady is going to be all right, but it always helps to have two great QBs."
Information from The Associated Press was used in this report.