All you have to do is go back and see what was said about Jackson when facing his peers at Nike Top 100 2 yrs ago. That is where best of best underclassmen compete for a weekend and Jackson was considered the best defensive prospect several analysts had seen in their time covering HS basketball.
He's an elite defender, a pro level shot blocker--but he can guard 1-5 and hedge/recover is insane. This is the type of athlete UK has not been getting and needed to start getting and finally are--Collins being another, Toppin/Fletcher too. Clarke/Boston obviously but too often the Cats were slower/less explosive team on the floor and this had to be addressed. Skill is great, but so many games at the NCAA level are won with players who can make plays that either keep you in a game or get you a possession by their individual talent/abilities. Think Poythress in '14 run on several plays be it Louisville, Michigan, Wisconsin, Marcus Lee's '14 run, and then think of a lot of plays where UK has lost recently due to a guy not being athletic/tough enough to get a rebound (Gabriel against KSU), Willis not choosing to sprint the floor to get back on defense, Travis not being able to do anything on either end at the rim against Auburn.
'21 will be Kentucky's best defensive team in years. Maybe not right away but usually when Cal says their defense sucks at this time, he knows they are very capable and good, it's just he wants them at an elite standard. When you have rim protection/eliminators, and length on perimeter you can make it very hard to score against and he's got to remember he's got 2 Lottery Picks on the wings his own guys face daily. He won't see that very often when playing opponents.