All of a sudden, the Green Team train is at full capacity again.

After a 2018-19 regular season that severely tested the notorious optimism of Boston Celtics fans, they’re are all smiles for the time being. If you listen closely you might even hear them whistling the tune of “Ain’t No Stoppin’ Us Now.” The Kyrie Irving replica jerseys are once again being worn with pride.

All it took was an Eastern Conference first-round sweep of a starless, scoring-challenged Indiana Pacers squad to get the mojo back in Boston. That’s how low the bar around here got after a frustrating six months, filled with team chemistry questions, that produced a 49-33 record and slotted the Celtics as the No. 4 seed in the East.

But now this 4-0 start to the playoffs has folks wondering if the Celtics have finally awoken from their underachieving slumber and are poised to maximize their lofty potential when it matters?

Sweeping anyone in the NBA Playoffs, regardless of a massive talent disparity, is a nice accomplishment. The Celtics should feel good about themselves. But the harsh reality is that they haven’t done or proven anything yet. That doesn’t mean they won’t at some point during this postseason, it just means they haven’t yet.

Boston’s sweep over the fifth-seeded Pacers certainly wasn’t emphatic. The C’s had to work. All four games were decided by 10 points or less and Indiana was within striking distance late in the fourth quarter in three of the contests. All that despite the fact that Indy didn’t have its all-star best player Victor Oladipo and on multiple occasions had bigger issues putting the ball in the basket than my 7-year-old’s YMCA team.

The Pacers played hard and tough, but this was a total mismatch. The four best players in the series all wore a Boston uniform. However, the tall tales of the vaunted “Playoff Kyrie” that have been circulating around have been a bit embellished. Irving averaged 22.5 points and 7.8 assists, but shot just 42.7 percent from the floor in the series. The star guard was good, not great. He’ll have to be much better when the C’s face the top-seeded Milwaukee Bucks in the Eastern Conference semifinals.

Still, there have been some very positive signs for the Celtics.

Six players averaged double-figure points, meaning Boston shared the ball. Big man Al Horford averaged a double-double (11 ppg, 10.3 rpg). Jayson Tatum, Gordon Hayward and Jaylen Brown were active, productive and efficient. Head coach Brad Stevens did an excellent job distributing minutes and seems to have found his main eight-man rotation. And the defense did play a role in those woeful Pacer scoring droughts.

When you consider the Celtics were widely forecasted to win 60-plus games this season and get to the NBA Finals, it’s hard to get excited about what they did to a Pacers team that everyone knew was going nowhere. Also, what they did to the Pacers is pretty much par for the course for top-four seeded teams in the NBA Playoffs.

The Bucks completed a sweep of the Detroit Pistons on Monday night, winning 127-104, while the second-seeded Toronto Raptors and third-seeded Philadelphia 76ers also closed out their series on Tuesday night.

There are the haves, and the have-nots. The Celtics are definitely one of the haves, but from now on they will only be facing fellow haves. Only Boston will have to go the remainder of the playoffs without the benefit of home-court advantage. Forty-nine win seasons have consequences.

The Bucks went 60-22 in the regular season. They have the likely league MVP in forward Giannis Antetokounmpo, who has only had to log 27 minutes per night in the playoffs because Milwaukee has been blowing out the Pistons. The Bucks are big, deep and they have shooters and scorers to surround Antetokounmpo. They’re a top-five team in the league in both offense and defense.

They also will have home court advantage in the series, which opens this weekend.

In other words, this will be a lot different than facing the Pacers.

This is when the playoffs really begin for the Celtics. If they can beat the Bucks, that would be proving something.