Dan Quinn taking over defensive play-calling from coordinator Joe Whitt Jr. is not a unique situation. Though coordinators are usually tasked with play calling on both offense and defense, a head coach might take that role.
In the Washington Commanders’ case, it's a sign of cracks firmly showing.
Though Whitt did not lose his job or his title, this is clearly a demotion. Quinn and his coordinator have a strong working relationship dating back to their time together with the Dallas Cowboys, and on the surface, that does not appear to have changed.
From an outsider’s perspective, it doesn’t seem as if defensive play-calling is the biggest culprit in the Commanders’ recent collapse.