I've heard this comment a lot recently. I don't really agree with it. He looks lackadaisacal sometimes when he's sizing up the play and looking for proper positioning. I'm not claiming he puts in ROR effort or has Reinhart IQ, but I think this criticism is over-stated. Also, he will improve in both areas over the next few years. Everyone is just panicking because they are comparing him to Matthews. Assuming they both stay healthy, I'll be surprised if Matthews turns out to have a materially better career than Jack, if at all.
Bottom line - I'm not worried about Jack. He's an elite offensive talent and not an elite defensive player. But defense is much easier to improve with experience and good coaching. One thing I'm certain of is that Jack is extremely competitive and will always try to improve his game and win.