It's not that Jack didn't understand the importance of defense, he was well aware that in any era, even the era of small ball, there was a qualitative difference between a team with defense and one without. He often provided defensive suggestions for Felton, took defense seriously himself, and it was even Jack who came up with the idea to choke off King James... but in previous games, he rarely took on significant defensive roles, as what the team needed him to do was primarily shooting.
The team hadn't performed badly on defense in the first and second quarters, so he naturally started looking into how to solve problems from the offensive end.
And at halftime, Bickerstaff even agreed with his approach... It felt like being enlightened.
