BOSTON -- Boston Celtics guard Marcus Smart had already run the gamut of emotions after lacerating his right hand with one ill-advised, frustration-filled jab at a glass picture frame inside a Beverly Hills hotel in late January. But, sitting alone in his suburban Boston apartment and forced to watch his teammates stumble toward the All-Star break from afar, Smart was certain this was the worst feeling of all:

Helplessness.

With every defensive miscue -- and there were an astounding number of them -- Smart screamed at his television, then wondered how things might be different if he was on the court. After games, Smart called his teammates and tried to talk them through the slippage he was seeing, desperate to provide the voice the Celtics so clearly missed on the court during his 11-game absence.

But words alone could not rescue this team.

"I'm sure they wanted me to just shut up," Smart said. "It's hard because there's nothing you can really do but talk. You want to be out there and get things straightened out but you can't. That was the hardest part."

Smart returned to the court when the Celtics reconvened following that much-needed All-Star break. And it's no coincidence that Boston has ripped off four straight wins, playing some of its most inspired ball in months with a motivated Smart back on the floor.

The Celtics bring their league-leading defense to Houston for a Saturday showdown with the red-hot Rockets (ABC, 8:30 p.m. ET).

To be certain, Smart is not the only reason for Boston's resurgence. Kyrie Irving has been downright spectacular and absurdly efficient since the break. Boston looks refreshed and refocused while feasting on lesser competition. But Smart is so tied to Boston's identity, particularly the defense that coach Brad Stevens prioritizes, and his return has undoubtedly given the Celtics a much-needed jolt.

Smart is living up to his nickname as the Young Game-Changer.

Boston's post-All-Star success has been validation for Smart. During his monthlong absence, his name danced in trade rumors and some Celtics fans were eager to ship out the longest-tenured member of the team for short-term bench help.

Boston has outscored opponents by an average of 15 points per game during this four-game win streak. Smart hasn't just restored order to the defense, he also has helped Boston's offensive rating skyrocket to 119.3 since the break. Only the Golden State Warriors (120.2) have been better in that span.

"It feels good," Smart said. "I think I'm one of the top defenders in this league. I think I'm a first-team all-defensive team guy. Jaylen Brown has been showcasing his potential, even Kyrie now, I think he's top two on our team in steals. We got guys like Terry [Rozier], Shane Larkin, those guys come in here and make plays. We've got a great defensive team.

"It's just we were lacking things and the All-Star break really helped us."

The lingering question, though: Can Boston sustain this level of play against an elite opponent like a Rockets team riding a 14-game winning streak?

The last time the Celtics and Rockets met, Smart's defense on Harden was crucial. Michael Dwyer/AP Photo

When these two teams met in December, Smart drew two offensive fouls on Rockets star James Harden over the final 7 seconds while helping Boston rally back from 26 down to stun the visiting Rockets. While Harden's vicious crossover on Wesley Johnson on Wednesday night has seemingly been on a loop on SportsCenter ever since, Smart is eager for another crack at defending the NBA's leading scorer and MVP front-runner.

"I love it," Smart said. "It's an opportunity for me to challenge myself on the defensive end and each and every day I'm up for those type of challenges. ...

"It's hard to stop guys in this league of his caliber. You can only really hope to slow them down. But my job is to make every shot that he takes tough. He's going to hit some tough ones, but if I keep contesting those shots, when it comes to the fourth quarter those shots get harder and guys' legs start weighing on them, and things like that."

During the first meeting between the two teams, Smart defended Harden on 34 possessions and allowed him to create just six points and three assists, according to Second Spectrum data. Harden turned the ball over four times against Smart and mustered just three total free throw attempts. In 35 possessions when defended by all other Celtics, Harden put up 25 points and six assists while attempting 12 free throws, per Second Spectrum.

Harden Vs. Celtics Defenders in Half Court In First Meeting Smart All Others Matchups 34 35 Points 6 25 Ast-TO 3-4 6-1 Free Throws 3 12 According to Second Spectrum

Asked if he occupies any real estate inside Harden's head after the last matchup, Smart shrugged and said, "I don't know. You've got to ask Harden about that one. I was just playing my game and so I don't know if I'm in his head or not."

Both Chris Paul and Clint Capela sat out that first meeting, something Stevens is quick to note when asked about Boston's ability to steal a win in December.

Top competition has brought out the best in Boston's defense this season. According to ESPN Stats & Information research, the Celtics have held the five teams with the best offensive efficiencies (Warriors, Rockets, Wolves, Raptors, Cavs) to an average offensive rating of 101.5 this season, more than 10 points per 100 possessions better than the average rating of those five teams this season through Thursday's games.

"Whenever we have defensive success it's all in terms of our energy," Celtics big man Aron Baynes said. "When we put the right energy out there on every single play that's when our defense looks good. I don't know if it's about playing the better teams or whatever it is but as long as we have good energy on defense and everyone is covering for each other then we look pretty nice."

The Celtics own a defensive rating of 96.8 when Baynes is on the floor this season. Among currently healthy players, Baynes owns the best individual rating in the league (Kawhi Leonard and Andre Roberson have better marks in more limited time this season).

Aron Baynes has provided a strong physical presence in Boston. Barry Chin/The Boston Globe via Getty Images

While Baynes plays a limited role (18 minutes per game), he has been Boston's most frequent big-man starter and provides the bruising physical presence the team has lacked in recent seasons. That gives more flexibility in how the Celtics dispatch Al Horford and limits the amount of time he must guard centers (at least until late-game situations when Boston prefers to go small with Smart on the court).

Among the 141 NBA players with at least 500 possessions defended this season, the Celtics have four players in the top 15 in points allowed per possession. Baynes tops that list (0.739), Horford is fourth (0.808), Smart is 11th (0.83) and rookie Jayson Tatum is 15th (0.832). Brown, who often draws the opponent's top wing scorer at the start of games, is 45th (0.88) but Smart is quick to point out how good Brown has been considering the level of competition he's assigned. Even Irving ranks in the middle of the pack (77th overall, 0.917).

The Celtics supplement that core with a group of willing bench defenders including Rozier and Larkin and rookies Daniel Theis and Semi Ojeleye. How does Stevens know when his team is locked in defensively on a given night?

"We look like we're more places than we are. We have a deceptiveness to us," Stevens said. "We're long, we have our arms out, we're in a stance, we're quick to react. For me it's always been, when that shot goes up, do we have five guys engaged in the rebounding and the guards flying over the top to get them.

"When you go back through some of our better possessions of the year, we are active, we're taking away the first option, we're taking away the second option and we fly in for rebounds. Easier said than done against a team like Houston, because the first and second options are hard to take away."

Even before his team got steamrollered on Wednesday night in Boston, Charlotte coach Steve Clifford raved about Boston's defense.

"When they get their defense set, their versatility, their length, their quickness defensively makes it hard," Clifford said. "They stay out of rotations, they have a number of guys who have the ability to defend multiple positions, and they're very disciplined in what they do."

Before the season, most pundits wondered if Boston's defense -- which ranked just 12th overall last season with a defensive rating of 105.5 -- might take a step back this season after trading away Avery Bradley and Jae Crowder. That chatter didn't sit well with Smart.

"I kinda thought that was a slap in the face to the guys that were on the team," Smart said. "For them to say we weren't going to have any defense -- those two guys, I love those guys, they're wonderful guys and they're awesome defenders, but I think I'm a top-notch defender, an elite defender and, like I said, one of the top defenders in this league. So it was kinda a slap to my face to me, and to my teammates, the guys that were already here. Like Terry and Al and those guys. We didn't take that lightly. We took that personally."

Back on the court, Smart seems to have a greater appreciation for being out there. His glass-shattering incident, which he insists stemmed from frustration over his play more than any social media drama that was evident that day, has put things in perspective for him.

On Wednesday night, the Celtics owned a 23-point fourth-quarter lead when Charlotte's Malik Monk got a hand on Smart's behind-the-back feed intended for a cutting Greg Monroe. Smart could have just shrugged off the misfire, but that's not what Marcus Smart does.

When Monk fumbled the ball a bit trying to head the opposite way, Smart lunged. We're talking swimmer-off-the-starting-blocks lunged, and managed to corral the ball while crashing to the floor. Moments later Monroe got his layup.

You get the sense that Smart doesn't want to endure that feeling of helplessness again.

"I love this game, I love to play," Smart said. "So when that kinda gets taken away from you, temporarily or permanently, you kinda start realizing how much you do miss this game and how much it is easy to take it for granted when you can't do that."