We will learn a lot about the Boston Celtics in Monday's Game 4 of their first-round series against the Miami Heat. This is the one that a true championship contender should win — or cannot afford to lose.

For inability or immaturity, Boston has had a habit of letting go of the rope in its last two title pursuits. As the path to an 18th banner clears in front of them, the Celtics would be wise to make quick work of an undermanned Miami squad. They know all too well how playoff fortunes can change on these moments.

In the 2022 Eastern Conference semifinals, with a chance to take a 3-2 lead against the Milwaukee Bucks, the Celtics blew a 14-point, fourth-quarter lead at home. A round later, again at TD Garden, they failed to close out Game 6 against the Miami Heat. Both series extended to a grueling seventh game, and by the time they reached the NBA Finals, the younger Celtics had no legs left against the Golden State Warriors.

"There were a few times last year where we kind of relaxed," Jayson Tatum conceded in 2023, when, with the Philadelphia 76ers looming, his Celtics had a chance to close out the Atlanta Hawks in yet another home Game 5. "We're trying not to make it tougher on ourselves, not to relax, and damn near go in there with the mindset that we're down 3-1 and we've got to win, rather than thinking it's over and they're going to give up. Because they're not. They're a really well-coached team, they've got great players, and they've got a lot of pride. Go in there with the mindset that we've got to win to survive, just learning from our mistakes last year that kind of made the road a little tougher for us, and I think we'll be ready."

The Celtics then coughed up another 13-point, fourth-quarter lead at home, extending their first-round series against the seventh seed to six games. The snowball of second-round Games 1 and 5 losses to the Sixers became a full-blown avalanche at the start of the conference finals, when Boston suffered three straight defeats to Miami. Most of the damage was done in TD Garden; all came in games Boston was favored.

Listen, winning in the NBA playoffs is hard, but it sure feels like the Celtics make it harder on themselves, and the ripple effects of each blown opportunity to advance compound, to be felt when it matters most.

Boston already had its hiccup in Round 1, losing Game 2 to the Jimmy Butler-less, eighth-seeded Heat at home. The Celtics let them get too comfortable at the 3-point line. Yes, they're a really well-coached team, they've got great players, and they've got a lot of pride, but Miami's talent disparity in the absence of Butler (sprained right MCL) and Terry Rozier (neck spasms) — both ruled out for Game 4 — is a canyon. The Celtics never trailed in their other two games, leading by a combined 61 points. All it took was effort.

In these playoffs, Boston ranks second in the East in both offensive and defensive rating, while Miami ranks second-to-last in the conference in both categories. Yet the Heat have a shot to even the series on Monday. This is what they do. Even undermanned, they force a full night's work from every opponent.

Boston is a double-digit favorite (-10.5) once again, according to BetMGM, and the line would be bigger if the Celtics did not have a history of punting golden opportunities. A path is clear: Win Game 4, and they have a shot to close out the series earlier at home on Wednesday; lose, and they are in for the long haul.

It is also becoming clearer on the other side of the opening round. The inconsistent Cleveland Cavaliers and Orlando Magic are locked into a 2-2 series that should go seven. Further down the road, the Bucks and Sixers are in big trouble, both trailing 3-1. The Julius Randle-less New York Knicks are looking like the most formidable challenger in the East, and Boston beat them four of five games in the regular season.

Even on the other side of the bracket, the Denver Nuggets, owners of a 3-1 lead against the Los Angeles Lakers, are now nursing Jamal Murray through a left calf strain. The second-seeded defending champions are trying to keep pace with the third-seeded Minnesota Timberwolves, who just swept the Phoenix Suns, and the top-seeded Oklahoma City Thunder, who can sweep the New Orleans Pelicans on Monday.

This is Boston's most talented team since the 2017-18 season, and that team was a lesson in the fragility of a contender. Injuries to Gordon Hayward and Kyrie Irving kept those Celtics from ever getting healthy. Younger players earned bigger roles. Chemistry frayed. Stars got weird. And the whole experiment failed.

It is a testament to the development of Tatum and Jaylen Brown that the Celtics are still here six years later. This is their team now. As Brown said earlier this season, "I'm in my athletic prime." Armed with Kristaps Porziņģis, Jrue Holiday and Derrick White at their side, the Celtics are as heavy a favorite as they have ever been on this run, and no team knows better how much harder each loss makes that road.

Another convincing win in Game 4 would represent a step forward for Boston. A loss? Same old Celtics.