Jaren Jackson Jr. is one of the best defensive players in the NBA, so seeing a drop-off in production when he is off the floor isn't earth-shattering. However, the Grizzlies' defensive rating difference was a significant gap when he was on the floor with Zach Edey vs when he was off the floor last season with Edey.
Memphis had a defensive rating of 109.6 when Jackson Jr. and Edey shared the floor, which would've made the Grizzlies a top-two defense if that number remained for the team entirely. The Grizzlies finished last season with a defensive rating of 113, which ranked 10th in the NBA, and was their defensive rating in lineups that included Jaren without Edey.
With just Edey and no Jaren on the court, the Grizzlies' defensive rating slipped to 121.7, which would've ranked last in the league. That can only mean that Memphis must play its starting frontcourt together as much as possible to ensure its defense doesn't drop off like it did often last season.
Memphis must pair Edey with JJJ to make it work on defense
The thing about numbers is that they can be inflated or deflated in a direction that doesn't consider other factors. Zach Edey is a good rim protector but struggles when switched out onto the perimeter, which is probably true for most big men over 7'0" in the NBA. Jaren Jackson Jr. is a great roaming defender, so their combination plays to their strengths on that end.
Another point to consider is that the Grizzlies' perimeter defense was lacking last season, and that figures to change this season with the integration of Kentavious Caldwell-Pope. When Ja Morant goes to the bench, the team could really put fear into teams with a combination of Scotty Pippen Jr., Caldwell-Pope, and Jaylen Wells on the perimeter.
Also, a negative defender like Santi Aldama was likely the player that Edey paired with in the front court when JJJ was off the court due to offensive spacing. Brandon Clarke is the much better defender, but can't space the floor on offense, so he spends more time playing with Jackson Jr. when the team alters their combinations.
The Grizzlies used this offseason to ensure their defense will be accounted for at all levels of the court, so Zach Edey's weaknesses will likely be minimized with the right pairings. They have the horsepower to produce a balanced effort on both ends of the court, so it's up to Tuomas Iisalo to trigger the right lineup combinations.