Isn't most of defensive ability just effort and trusting your teammates? I think with a good coach that has a system the players trust, we could get better defensively really fast.
At the high school level? Yes. At the NBA level? No. The good NBA teams can hide the defensively deficient if they have some stellar defenders (bulls hide boozer with noah, etc.). Good defenders at the NBA level are a step quicker than the offensive threat, with a strong and long frame, quick and responsive knees, great lateral quickness, a jump in their step, a large wingspan/ standing reach, and loose hips that allow them to dip or turn on a dime. As for god-given ability to defend, Jabari only has a large frame and wingspan--like Kanter. Sure, Jabari can become a better defender than he is now with learning his spots, knowing and trusting his team, and applying effort, but he'll never be able to fake the body he doesn't have, and neither will Kanter.
Drafting Parker makes our starting line:
Burke --incredibly weak defender
Hayward --good size and quickness, but can still get burned by those w/ quick first step. Doesn't help the helper.
Parker -- slow to react. doesn't have the ability to contain anyone, which means Melo, Durant, LebRon, etc. will light us up.
Kanter-- see Jabari. Has gotten a lot better at knowing when to rotate, and isn't taking possessions off, but has the kind of slowness that is blood in the water for faster players, and is constantly exposed because NO ONE on our team helps the helper (the guy who rotates to the leak, which is often Kanter so Favors can protect the rim).
Favors-- is really just an above average defender at this point. He has great length, and body for defense, but is still allowing just 1 point less than Kanter is per 10 games.
If we draft Jabari, we will need to bench Trey and possibly Kanter, or trade for better defenders so there is some kind of balance on the team. I don't think most people realize how god awful of a defender Trey Burke is. He is where most of the leaks occur from. Kid makes Al Jefferson look like DPOY, but people don't comment on it because it's not always at the rim, so it's less noticeable.