By Matthew Geagan, CBS Boston

BOSTON (CBS) — All it took was a little playoff basketball for the Celtics to finally arrive. They’re playing their best basketball when it matters most, completing a rare first-round sweep of the Indiana Pacers on Sunday.

Now they get to enjoy a week off before taking on their next opponent, and they’re going to need it. That next opponent is 99.9 percent certain to be Giannis Antetokounmpo and the top-seed Milwaukee Bucks, who are up 3-0 on the Detroit Pistons at the moment. That series should end Monday night, though the second-round won’t tip off until sometime this weekend.

Rest up, Celtics. It’s not getting any easier from here on out.

That’s not to say their sweep of the Pacers was easy, because it certainly was not. The gritty Pacers battled hard on the defensive end, but were no match against Boston’s talent without star player Victor Oladipo. The Celtics were far from perfect throughout their four wins over the Pacers, and needed to find different ways to notch each victory.

But while just about everyone picked the Celtics to win the series, no one picked them to pull off a sweep. And whenever you sweep an opponent in the playoffs, you had to do many things right. Breaking out the brooms against Indy has the Celtics brimming with confidence.

“It’s just the beginning for us,” said Gordon Hayward, who led the way with 20 points off the bench on Sunday. “We still got a lot of work to do.”

“We’re clicking at the right time,” Jayson Tatum said after an 18-point effort in Game 4. “We look like the team everybody thought we would be. It took some ups and downs for us to get here. I like the way we look.”

The Celtics did a whole lot right over the last week, which should make everyone feel much better about their chances going forward:

– Even with a 37-point outburst in Game 2, Kyrie Irving wasn’t a monster for most of the series. But he was Playoff Kyrie when he needed to be and gave the Celtics plenty of clutch moments in crunch time.

– Tatum has embraced his role as Boston’s No. 2 behind Irving, averaging 19.7 points against the Pacers. Tatum shot 50.9 percent from the floor and 53.3 percent from three-point range for the series, and didn’t look timid with the ball in his hands.

– Al Horford’s defense was strong to very strong throughout the first round. And though he didn’t put up big offensive numbers, he continues to play a key role in making it all work, getting guys open with screens and spacing the floor.

– Jaylen Brown is playing his role and not trying to do too much, contributing with some solid defense and scrappy baskets.

– The bench has been incredible, led by the re-emergence of Gordon Hayward. The guard scored nine of his 20 points in the fourth quarter on Sunday, and he finished 7-for-9 from the floor. When Hayward plays like a stud off the bench, it trickles down to the rest of Boston’s reserves. Marcus Morris had 18 points on Sunday and Terry Rozier finished with 11 off the pine. All of a sudden, the Celtics are the ridiculously deep team we were promised ahead of the season.

– In what feels like a microcosm of the regular season, the Celtics saved their best basketball for the end of each of their four wins against the Pacers. They locked down in the third quarter of Game 1. They staged a double-digit comeback in the fourth quarter of Game 2. Boston went on a furious run in the final six minutes of Game 3. The win was never really in doubt late Sunday afternoon, as the Celtics turned a one-point lead into a double-digit advantage in the fourth quarter before hanging on for the clinching victory.

The Celtics showed a closer mentality throughout the series, something that escaped them for large stretched during the regular season. As frustrating as it is, they have indeed flipped the switch this postseason. They look like a completely different team than the one that couldn’t seem to get out of its own way from October to March.

“The regular season is done with. We’re not that same team,” Morris said confidently on Sunday. “Guys understand the goal ahead and it shows on the court. We pick each other up every game. Every game someone else is stepping up. Our depth has been a strong point for us, and we’re just getting the wins.”

The Bucks, however, are not the Pacers. They aren’t even the same Bucks that the Kyrie and Hayward-less Celtics sent home in the first-round last year. They have an actual coach leading them, Giannis took his game to an MVP level, and Milwaukee was the best team in the Eastern Conference from start to finish. They took two of three from Boston during the regular season. The Celtics cannot afford to make the same mistakes they made against the Pacers when the East Semis tip off. Milwaukee’s offense is not going to fall asleep for long stretches or disappear like Indiana’s did.

It’s just going to get tougher and tougher for the Celtics. But that’s how it should be when you’re one of the final eight teams left standing in the NBA. At least they have plenty of time to prepare for their next bout.