The Boston Celtics are a different team when Marcus Smart is on the floor.

If there was any doubt about that, the first two games of the Eastern Conference Finals should probably put it to rest. After missing a Game 1 loss against the Miami Heat with a foot sprain, Smart returned to the lineup on Thursday with 24 points, 12 assists, nine rebounds, five threes, three steals and a block. He was plus-31 in 40 minutes.

No other Celtics starter was better than plus-16 in a 127-102 rout that wasn't even that close.

The differences between Tuesday and Thursday's games were clear, and it was easy to trace several of them back to Smart.

For one, Boston was simply outworked in a 39-14 third quarter of Game 1. When it became clear Miami was bringing an effort level the Celtics couldn't (or wouldn't) match, the wheels fell off. Settling for jumpers, turnovers (Jayson Tatum had six in that frame alone), you name it. Boston lost the battles for physical and mental toughness.

"They just came out and kind of punked us in that third quarter," head coach Ime Udoka said.

Thursday, it was obvious in the second quarter that that wouldn't happen again.

"We just wanted to come in and be the harder playing team tonight," Smart told reporters after the game. "We said we were going to get the loose balls, cut the turnovers and let our defense do what we do."

All those boxes were completely filled. And Smart was often the one holding the metaphorical pencil. As he has been all season, Smart set a tone in Game 2, particularly with his on-ball defense and tenacity on switches.

For most teams, switching 1 through 5 would be asking for a disaster. For the Celtics, when healthy, it's what sets them apart from everyone.

Smart, Tatum and Jaylen Brown can hold up against just about anyone but the league's burliest 5s. Al Horford (who also returned after missing Game 1) and Robert Williams III are some of the game's best bigs at scrambling on the perimeter against guards and wings.

After Game 2, Udoka said, "We wanted to keep bodies in front of bodies," and that's exactly what they did. That's what they've done better than anyone since Jan. 1. Over that stretch, Boston's first-place defense (by points allowed per 100 possessions) had around the same distance between it and second as the distance between second and 11th.

While all five of the starters were an integral part of that success, these two games against Miami show that the Defensive Player of the Year may be the real key.

Tatum, Brown and Derrick White are stellar perimeter defenders (what a luxury it must be to have that many who can be described that way before you get to the DPOY), but Smart plays with that edge and relentlessness against stars that made careers for players like Tony Allen and Bruce Bowen.

Jimmy Butler had 41 points on 19 field-goal attempts in Game 1. Thursday, his total for free-throw attempts was cut by more than half and his points leveled off at 29. He was still good, but that's a manageable line. And though it took a team effort to get him there, Smart's one-on-one opportunities made Butler work as much as anyone's.

"I love going up against Jimmy any time I can," Smart said. "As a defensive player, as a competitor, he’s going to make you work, he’s going to make you better."

Smart has been taking matchups like this personally all season long, and the results loudly speak for themselves.

When Smart was on the floor in the regular season, the Celtics allowed 106.1 points per 100 possessions (a mark that ranked in the 92nd percentile) and outscored opponents by 9.7 points per 100 possessions. That point differential is around that of a 63-win team (it was around that of a 50-win team when he was off the floor).

In other words, the Celtics are really good without Smart. They play like a juggernaut with him.

And it's not entirely because of his defense.

No one's accused Smart of being a great shooter, but volume (he took 5.1 attempts per game this season) can often have the desired effect of accuracy. Just knowing he'll take the shot and make just enough forces defenses to pay attention to him outside. And in the games when he's on, as he was Thursday, Boston becomes exponentially more difficult to slow down.

Smart hit a game-high five threes (to go along with his game-high four deflections and three steals).

More importantly, his ball-handling and passing hide one of the very few flaws Tatum has. For all Boston's superstar can do, there are moments when his handle seems a little shaky. And reserving some of that responsibility for Smart frees up Tatum to do more attacking off catches.

Thursday, Smart's 12 assists had the internet buzzing.

"It feels great," Smart said of being able to play the 1 (after years of working alongside Kyrie Irving and Kemba Walker). "That's who I am. The whole world is seeing what I can do at the point guard position."

And what he does, on both ends of the floor, may make the Celtics more than the Heat can handle.

As Jeff Van Gundy pointed out during ESPN's broadcast of Game 2, Boston is bigger, more athletic or both at almost every spot on the floor.

Just think about each matchup. Smart versus Gabe Vincent, the former has a 20-pound advantage. Brown is an inch taller and quicker laterally than Max Strus. I'm not going to argue Tatum has an edge over Butler in terms of toughness, but he is a bit taller and has a longer wingspan. P.J. Tucker is a bulldog, but he's much smaller than Horford. And finally, Williams can match Bam Adebayo's explosiveness, while Horford sort of provides the "playmaking big" qualities.

Miami is talented. When the Celtics are healthy, they're talented, big and can embrace most of today's philosophies. Switchability, particularly from Smart, Brown and Tatum, allows them to play positionless when necessary. Horford and Williams can bully opponents inside without compromising that.

Of course, this series is tied, 1-1. Another monster performance (or two) from Jimmy could alter our perspectives again. But right now, Boston's starting lineup (and really, first seven or eight guys) seems perfectly engineered for today's game.

From the front office to the coaching staff to the players, plenty of credit should be doled out for that. As it is, don't forget about Smart, the on-court conductor (as if performances like Thursday's would even make that possible).