Trent McDuffie fits into the Rams' secondary as the centerpiece of a complete overhaul — he and fellow Chiefs import Jaylen Watson were brought in together to plug what was the defense's biggest hole. DC Chris Shula called McDuffie "such a great cover guy" who's "so much more than that," and plans to use his versatility as a leader among the upgraded DB room (from the Trent McDuffie page).
Here's the key synergy with the Myles Garrett trade: the two moves are designed to work together. Shula's scheme uses Garrett to dictate double-teams and generate instant pressure, which shortens the time McDuffie and the secondary have to hold coverage — making their jobs easier (from the Chris Shula page). One NFL coach even told The Athletic that McDuffie and Watson "will impact their defensive unit more than Myles," since CB was the Rams' glaring weakness while edge rusher was already a strength. So McDuffie isn't just a sidekick to Garrett — he might be the more impactful addition for the secondary's day-to-day performance.
On Fantasy Football Today, they noted that the Rams defense "could be really, really dominant" with these additions — and that a dominant defense could mean more offensive possessions and fantasy production for the Rams' skill players too.