You might wonder why the Boston Celtics are so eager to get Marcus Smart back, considering he is one of the NBA’s worst high-volume shooters (if not the worst) and he hasn’t seen the court in six weeks.

But then you wouldn’t understand Marcus Smart.

Rust doesn’t much bother Smart, partly because his jumper always needs oil at the joints, but mostly because he is the rust. He finds your weaknesses and sticks to them until you’re fully corroded. Hard as you may try to touch up the scratches and dents, he’ll spread like an iron oxide into your offense.

So, the news from Celtics coach Brad Stevens on Tuesday morning — that Smart was physically cleared by his doctors in New York to return from his right thumb injury, pending a pregame workout setback — comes with little expectation from anyone that Boston’s sixth man won’t be on the floor for Game 5.

Don’t forget these same Bucks called him “trash” earlier this season. He’s champing at the bit.

Two Milwaukee Bucks have torched the Celtics in their first-round series, which enters Tuesday’s tilt in Boston even at two games apiece: Giannis Antetokounmpo and Khris Middleton. The C’s can’t do much about the former, who even Stevens will concede is un-guard-able, but Smart can frustrate the latter.

Save for one bully-ball stretch with Greg Monroe, all of Boston’s best lineups in the series have featured Al Horford at center. But because the Celtics are so undermanned — without Smart (until now), Kyrie Irving, Gordon Hayward and even stretch big Daniel Theis — Stevens has had precious few options outside of the lineup of Horford, Marcus Morris, Jayson Tatum, Jaylen Brown and Terry Rozier that has outscored the Bucks by almost 10 points per 100 possessions. Lineups featuring both Rozier and backup point guard Shane Larkin have been disastrous. Stevens tried Guerschon Yabusele and even Abdel Nader before finding some success with Semi Ojeleye for stretches in Game 4.

But Smart is miles ahead of all three of those rookies. Because Smart hasn’t played much since the Celtics lost Irving for good this season, there aren’t huge sample sizes for the small-ball lineups Stevens will use going forward, but almost all of them have been wildly successful in limited stints.

Lineups with both Smart and Rozier on the floor outscored opponents by four points per 100 possessions in 824 minutes this season. And lineups with Smart and no Rozier or Irving were even better, albeit mostly against second units. The point is: Smart changes the game. The Celtics were better on both offense and defense with Smart on the court this season for 30 minutes a game.

Smart’s conditioning may not be ready to handle that load, although he’s been working out without contact for a while now. (He’s also been shooting without issue for more than a week, which makes a setback unlikely prior to Game 5.) But whatever minutes he can give them will be invaluable, because Stevens has options now. Smart can handle the ball either alongside Rozier or with him on the bench, and the Celtics can rotate Tatum, Brown and Morris around either or both of them to maximize their Horford-at-the-center minutes, all but eradicating Larkin, Ojeleye and others from the rotation.

Mostly, though, Smart allows Boston to switch everybody. Against the Bucks this season, he split much of his time between Middleton and Malcolm Brogdon, limiting both, but he also enjoyed success defending Antetokounmpo in spots. Brown and Tatum are capable as tentpoles of the league’s best defense, but moving Smart onto Middleton puts their best defender on their biggest solvable problem of the series while pushing the length of Brown and Tatum onto guys like Jabari Parker and Thon Maker, who have also got comfortable shooting over smaller or less athletic defenders in Milwaukee:

This is to say nothing of the little things he does, like baiting James Harden into blowing a game. Nobody will be comfortable when Smart is on the floor, and that’s why the Celtics aren’t worried about how he’ll fit now that he’s been cleared. He doesn’t fit. He tears everything to pieces.

“He hasn’t played in a month and a half,” said Stevens, “so this is an opportunity to come in and do what he does best, and that is impact the game with his energy, his physicality, his savvy and his skill.”

Smart has been watching from the sidelines as the Bucks have had their way on offense against his Celtics, all while learning that his mother was suffering from cancer and that she wants nothing more than to watch her son go to battle in the playoffs once more. So, it’s no wonder Smart told The Athletic’s Jay King last week, “That’s why we’re going to keep fighting until we can’t fight no more.”

This is the same guy who was so upset by a loss to the Los Angeles Lakers earlier this season that he nearly tore a thumb tendon punching a picture frame. He missed three weeks in January as a result, only to return to average 10.8 points, 5.6 assists, 3.1 rebounds and 1.6 steals off the bench for eight games before injuring the same thumb diving for a loose ball — and then played another 16:30 with it.

Rust never sleeps.

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Ben Rohrbach is a writer for Yahoo Sports. Have a tip? Email him at rohrbach_ben@yahoo.com or follow him on Twitter! Follow @brohrbach

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