Brad Stevens absorbed a lot of hard lessons in his first season with the Celtics in 2013-14. When the coach took his team to the playoffs for the first time in 2014-15, the aches doubled. The Celtics were swept in the first round by Cleveland, a team that included Kyrie Irving, the offensive wizard who led the Celtics to their 16th straight win Monday night in Dallas with a 47-point performance. But in April 2015 the lessons weren’t about offense. “We kind of snuck into the playoffs late in the season, and one of the things that was pretty obvious was that we weren’t very versatile defensively,” said Stevens. “We were really hard-playing, we had a lot of guys who were able to do a lot of things, but we weren’t able to switch quite as much.