Most truly great offensive players get lazy on defense as they age. Yes it isn't a good sign that Irving has never tried defensively, but it is incredibly taxing on the body to go full bore on both ends of the floor for any real length of time. Even the all time greats focus less and less on defense when they get older so they can remain fresher offensively (Jordan, Bird, Magic, Kobe, James, even Durant has started that). Leonard is still pretty young in the scheme of things, and he doesn't quite have the all around offensive load that Durant and James have, but over time even Leonard will focus less on one side of the floor to remain fresher for the other. It is the nature of the beast.
I bring this up, because I'd rather have the killer on offense then a guy that works so hard to improve defensively and that he becomes quite average offensively. For the Celtics to be a real contender, they need Irving to be an unstoppable force offensively. If he can remain that offensive force and improve his defensive effort and ability that would be great and would start putting him into the all time great category, but if not, if he stays the offensive weapon I'm ok with that. Ideally I'd like Irving to become a bit more like Iverson on defense i.e. a guy that picks his spots, disrupts lanes, etc. If he can become that level of defensive player and not lose anything offensively then Boston could end up being a title winning team with the Irving/Hayward duo being supplemented by Brown, Tatum, LAL, Smart, etc.