BOSTON -- Marcus Smart once dislocated two fingers diving for a loose ball in an otherwise unremarkable summer league game, so no display of hustle should be that surprising. Smart simply does not know how to throttle down on a basketball court.

But just in case anyone needed a refresher, there was Smart on Friday night in Philadelphia, lunging at Richaun Holmes as the Philadelphia 76ers center tried to advance the ball from the backcourt in the second half of an exhibition game.

Smart, giving up 6 inches, got a hand on the ball and clung to it as Holmes tried with two arms to prevent Smart from wrestling it away. Eventually, as the two do-si-doed, the ball popped loose near the free throw line. Smart immediately launched himself between the legs of T.J. McConnell and snared the loose ball. Smart even had the presence of mind to roll on his back and somehow feed teammate Terry Rozier, who then lobbed an alley-oop that rookie Guerschon Yabusele finished with a layup.

When the Boston Celtics convened to watch film of the games two days later, coach Brad Stevens made sure to spotlight the sequence.

"Now you guys that weren't here before know why we all love Marcus," said Stevens. "He does things that other guys just don't do. And that's a great tribute to him."

Marcus Smart is the longest-tenured player on the Boston Celtics, even though he's only 23 years old. John Geliebter/USA TODAY Sports

Smart, the No. 6 pick in the 2014 draft, is entering his fourth NBA season. On a Celtics roster that underwent an unprecedented overhaul this summer, he wears the badge of "longest-tenured Celtic," even if he finds that notion rather preposterous. Still, as one of just four returning players, Smart is a small slice of familiarity on Boston's new-look roster.

Smart, too, has a new look. He dropped 20 pounds this summer with help from a private chef and a diet heavy on Mediterranean food ("Dates are great; I love them," said Smart). He acknowledged that he battled back pain at times during last year's playoff run when his weight climbed. Smart is noticeably slimmer, and there's a renewed explosiveness to his game -- just watch this video from recent post-practice, one-on-one work when Smart soars for a slam as rookie Jayson Tatum scrambles to defend:

Maybe not coincidentally, the 23-year-old Smart is eligible for an extension on his rookie pact before the regular season tips on Oct. 17. The Celtics must decide if it's worthwhile to give Smart a hefty payday now or run the risk that a solid season could further elevate his price tag and allow rivals a chance to bid for his services.

While league sources indicate the Celtics are interested in an extension, Smart confirmed a Yahoo! Sports report Monday that suggested that neither he nor his agent have received a formal offer from the team yet.

"Me and my agent haven't heard anything from the Celtics," said Smart, who pointed out how other players from his draft class have inked big-money extensions. "You're seeing everybody else's numbers and things like that, so we're just kind of waiting."

Despite the lack of talks, Smart said his preference is to be in Boston into the distant future.

"I would love to be here long term. I hope I'm here long term," said Smart. "I love the organization, I love the fans, I love everything the Celtics are about."

For Boston, an extension might ultimately hinge on what the market sets for Smart's value. It seems unlikely Boston would be willing to pay Smart the sort of money Gary Harris got from the Denver Nuggets (4 years, $84 million), but there's a more palpable number for a Celtics team that's set to live above the cap for the foreseeable future. Staring at an eventual climb back into the luxury tax (and, further out, repeater penalties), the Celtics must still be responsible with their spending, especially considering that many rivals won't have much to spend on the open market this summer.

Regardless of whether the two sides come to an agreement before next week's deadline, there's an obvious urgency to Smart's play this preseason, and a clear desire to take a step forward this year.

Smart has heard all of the complaints about his play. It's part of the reason he got off social media this summer. You don't need to remind him of his shooting struggles -- he has shot 35.8 percent from the field for his career, including a cringe-worthy 29.1 percent beyond the 3-point arc. He spent part of his offseason working out in Denver with Chauncey Billups trying to figure out ways to harness his unique power guard skills.

But Smart also knows how much he impacts winning, and box score stats cannot fully quantify his impact. So when Stevens highlighted Smart's hustle during that recent film session, it resonated with Smart.

"It feels tremendous," said Smart. "I do a lot of things that don't show up in the stat sheet. And those things, you can't teach. This league is so focused on scoring the ball, but they don't look at the other things, like diving on a loose ball when it's a close game, a one- or two-possession game, getting your team another possession. Or ripping the ball away from somebody or taking a charge."

The way Smart slimmed down this season, some wondered if he'd still be able to guard bigger and stronger players. Smart found the suggestion that he might struggle amusing.

"I definitely laughed at that," said Smart. "I've been guarding bigger guys ever since I was in college, same weight I am now when I was in college. That's never going to change; my strength is never going to change. Just because I slimmed down, that's to my advantage. I can move a lot faster now, in addition to my strength."

During Monday's Celtics-Sixers rematch at TD Garden, Smart put his grown-man strength on display again. Sixers rookie Markelle Fultz, the No. 1 pick in June's draft, drove hard down the right side of the lane and tried to spin into the paint. A backpedaling Smart ripped the ball out of Fultz's hands as he went up for a shot and, balancing on the end line, Smart flipped the ball to a teammate to start a break the other way.

Smart's hustle plays aren't lost on his new teammates.

"He's a scrapper. He's a competitor. He's a bulldog," Gordon Hayward said of Smart. "And it's fun to play with guys like that. He's always going to make the energy play, the hustle plays, and I think, in the [first Philadelphia] game we had where he dove on the ball and he got the ball and we got a layup, those are like game-changing plays."

Smart's offensive play has been solid through Boston's first three preseason games. He's averaging 10.7 points per game on 54.5 percent shooting overall. He has connected on 7 of 12 3-point attempts (58.3 percent). Even in off-day workouts, Smart simply looks more confident shooting the ball.

Smart has started only 72 games in his first three NBA seasons and has said that he is more comfortable coming off the bench for head coach Brad Stevens. Brian Babineau/NBAE/Getty

Stevens hasn't committed to what Smart's role will be this season. He covets his experience and ballhandling abilities off the bench but has also said the Celtics must be flexible, and there could be matchups in which Smart makes sense with the first unit.

The big question is what kind of player will Smart eventually be? Is he the sort of player that can start opposite Kyrie Irving in Boston's backcourt or will he live in that Sixth Man, energy-giving role?

There are metrics that love Smart, such as FiveThirtyEight's CARMELO, which categorizes Smart as a "future All-Star." One of the metric's comparable players is beloved former Celtics guard Dennis Johnson. Another is Avery Bradley, Boston's former all-defense starting shooting guard who was traded to the Detroit Pistons this past offseason.

As the Celtics search for a new identity with their overhauled roster, Smart's play could go a long way toward shaping how this team is viewed, especially on the defensive side. It's clear Stevens has a special appreciation for what Smart brings to the floor.

Boston's front office must decide just how much it's willing to pay for what the box score can't always tell you about Smart's impact.