WALTHAM -- The left-handed layup in transition was pretty, yes, but the more important moment might have arrived after practice, when Terry Rozier discussed one of his trouble areas with Boston Celtics head coach Brad Stevens.

During the first three days of summer league practice, Rozier said he has noticed a major difference in defending off-ball screens against professional players. Marcus Smart and James Young aren't 10-year veterans, but they have used their NBA experience to teach Rozier a few lessons.

"Just stay attached to my man," Rozier said after practice Friday, "because you're going against guys that have been in the system. Marcus Smart and James Young, they know how to use screens really well; they know when to fade it, things like that. I have to practice staying attached. I talked to Coach Stevens today, so I've got an understanding of how to get better at it."

"Just basically talking about player and location," Rozier added. "How do you know how to guard people? You know when to go under screens, when you're guarding a certain player, you've gotta trail him and stay on his hip, when you're guarding a (Stephen) Curry or something like that."

Despite aiming constructive criticism at himself, Rozier arrived in the NBA with a strong defensive reputation. At the very least, he has also shown glimpses of his offensive potential at summer league practices. During a brief scrimmage reporters could see Friday afternoon, the 21-year-old drove down the right side of the court in transition, beat his defender toward the middle of the court, and lofted a left-handed scoop shot through the rim. In moments like that, he shows off the "burst" Stevens has raved about since draft night.

"I don't want to overdo it. We've played 18 total minutes of scrimmaging," the coach said Friday. "His speed is the highest level. Once we start going up and down in the fullcourt, he did a lot of things that kind of took you aback."

Before taking another breath, Stevens pumped the breaks on the praise.

"At the same time, he's got a lot to learn and a lot to work on," he said. "But I know he'll do that. I've got no question about that."

At summer league, Stevens has one hope: daily improvement. According to Rozier, the coaching staff has also preached to the squad that it should strive to be "the toughest defensive team in summer league." With Rozier, Marcus Smart and Phil Pressey setting the tone on the perimeter, plus mobile bigs like second-round pick Jordan Mickey, this team has the potential to make opponents uncomfortable while flying up and down the court.

For the rookies, this is all new: the offensive system, defensive schemes, shorter shot clock and everything else. Still, Rozier said his focus isn't on establishing himself or even getting comfortable with his new teammates.

"I'm trying to win," he said. "That's the most important thing: contributing to winning. We've got a goal to be the toughest defensive team in summer league, and we're trying to win. So I'm trying to be a part of that. I'm not trying to break up into my individual play. I'm just trying to contribute to winning."

Young getting older

During the open part of the scrimmage, returner James Young canned two out of three 3-pointers and pressured rookie R.J. Hunter into a low-percentage shot from the baseline.

Plenty of offseason buzz has surrounded Young's transformed body, and Stevens notices the improvement.

"Way different," the coach described his 19-year-old shooter. "Just from a physical standpoint, he's bigger, stronger. I think he's got a much better feel for how the game is played. I think he has a much better understanding of where he can be impactful on both ends. I'm encouraged by his play and I'm intrigued to see him play.

"There's been good competition amongst our young guards in there, and I think that will only continue. I actually wish we had 10 days of practices and two days of games. But we'll see how it goes once we start playing all those games."

Stevens talks free agency

The Celtics haven't acquired a big star in free agency, but have reached agreements to bring back Jonas Jerebko and Jae Crowder while adding newcomer Amir Johnson. As of Friday, 11 returners have guaranteed contracts to return for another season, providing the potential to build on a strong regular-season finish.

Stevens surely wouldn't have minded a talent influx, but routinely stresses the importance of continuity.

"I just think in general terms, the more continuity, the better," he said. "Obviously we've talked about the idea or the positive concept of corporate knowledge, and being able to draw back from experiences that you've lived together in the past. So obviously it's great when you've got guys like Avery (Bradley), Kelly (Olynyk), Sully (Jared Sullinger), that have been here the whole time, and that you can draw back on all those experiences. But we've got guys in (the gym) right now, that are going to be on our roster, that haven't been here. You just kind of blend it all together, and hopefully you get good leadership from those guys that have been there the whole time as far as helping, communicating and pulling guys along."

Sidestepping a question about whether the Celtics are done in free agency, Stevens suggested that's the concern of the front office.

"I'm not going to speak in any terms of certainty, right?" he said. "Obviously I can't comment on all the free-agent stuff, but I feel good about the direction we're headed."

Added the coach: "There's a number of factors that go into play in free agents deciding where they ultimately land. Some of those factors are within your control, and you can position yourself to have a chance. Some of them are way out of your control. And I think the biggest thing is that I feel really good about our environment. I think people will really enjoy playing with this group of people. I don't think there's any question about it. We've got guys that like the game, that work at the game. They want to be good. And we have to focus our attention on those guys and helping them become the best we can be."