The Celtics’ elite defense will face Giannis Antetokounmpo and the Bucks in the East semifinals.

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The battle-tested Boston Celtics find themselves in the conference semifinals for the fifth time in the last six years, hosting a formidable foe in the defending champion Milwaukee Bucks, which enter Game 1 likely missing one of their top players in three-time All-Star Khris Middleton, who is reportedly expected to miss the entire series.

The Bucks’ second-leading scorer, Middleton sprained the medial collateral ligament in his left knee in the fourth quarter of Milwaukee’s Game 2 loss to the Chicago Bulls on April 20. Since the 2017-18 season, Middleton has faced the Celtics in two playoff series, shooting 51.6% for an average of 22.4 points per game.

Boston, meanwhile, heads into this series well-rested with a relatively clean bill of health after quickly dispatching the Brooklyn Nets in four games to become the only team to sweep through the opening round.

As NBA.com’s John Schuhmann points out, the Celtics have simply been the best team in the NBA for the last three months, running off victories in 26 of their last 32 regular-season outings, only to step into the postseason nearly peaking. Robert Williams III held up that process somewhat after undergoing knee surgery in March, but he returned for Boston’s Game 3 win over the Nets in the first round. Celtics’ coach Ime Udoka admitted that Williams’ layoff negatively affected his conditioning, but that’s an area they’ve worked to improve over the course of nearly a week as the club awaited its next opponent.

The Celtics eliminated the Nets on the strength of physicality and suffocating defense on superstars Kevin Durant and Kyrie Irving. But they face a more arduous task in trying to slow down reigning Finals MVP Giannis Antetokounmpo, who is 7-5 in the postseason against Boston, averaging 26.8 points, 10.1 rebounds and 1.5 steals. Antetokounmpo thrives on physicality, but with co-star Middleton expected to miss time, the Bucks need other players to step up such as Jrue Holiday, Bobby Portis and Grayson Allen, who has been a lights-out 3-pointer shooter thus far in the postseason.

3 Things to Watch

1. Defending Antetokounmpo: Boston opened up the playoffs against Durant by throwing the kitchen sink at him early, and that definitely affected his rhythm. In just the first half of Game 1, the Celtics defended Durant with Defensive Player of the Year Marcus Smart, Jaylen Brown, Jayson Tatum, Al Horford and Grant Williams, limiting one of the game’s all-time great scorers to just seven points on 2-for-10 shooting.

Expect a similar defensive plan against Antetokounmpo, who will likely be less affected by Boston’s physicality than Durant was. Tatum matched up against Durant as the primary defender for much of the opening round, but Antetokounmpo outweighs the Celtics’ wing by more than 30 pounds. Either way, you can expect plenty of double teams to be flying at Antetokounmpo, who will need to find open shooters. The two-time Kia MVP needs to keep an eye out for Brown, too, as he’s averaging 2.5 steals in the playoffs.

2. Portis’ contributions: Portis joined Milwaukee’s starting lineup after the injury to Middleton, and he averaged 16 points and 13 rebounds across his first two starts of the postseason, hitting 50% from the floor and 42% from 3-point range as the Bucks cranked up their defense to compensate for the expected dip in scoring. Bucks coach Mike Budenholzer said “when we’re good defensively, that usually bleeds into our offense,” and added the team has “leaned pretty hard” on that end with Middleton out of commission.

That certainly needs to continue against a Celtics team with so many scoring options led by Tatum and Brown. Four of Boston’s five starters average double figures in scoring. So, Portis needs to step up to help Antetokounmpo and Holiday match some of that production.

3. Elite two-way Tatum: Tatum’s confidence must be soaring after his performance in the opening round against fellow superstar Durant. The three-time All-Star averaged 29.5 points, 7.3 assists, and 1.8 steals in the first round, but he’s facing a stiffer defense in the semifinals led by Antetokounmpo and Holiday with Brook Lopez anchoring the interior.

The positive for Tatum is he’s improved tremendously this season as a playmaker and can facilitate with confidence if Milwaukee locks him down because of Boston’s strong supporting cast of shooters. Tatum and Brown are already difficult enough to slow down, but Smart, Horford, Grant Williams and Payton Prichard have been deadly from deep so far in the postseason.

Number to know

18.2 — The Bucks held the Chicago Bulls to just 96.5 points per 100 possessions in the first round, 18.2 fewer than the Bulls scored in the regular season. In the 26 years for which we have play-by-play data (416 teams), that was the biggest drop-off in offensive efficiency from the regular season to the first round of the playoffs. Games 1 and 3 were the Bulls’ third- and second-least efficient games of the season, respectively, topped only by a game back in November.

* Last year’s Bucks were responsible for the fifth biggest drop-off, having held the Miami Heat to 15.2 points per 100 possessions than they scored in the regular season.

The Celtics had the league’s top-ranked defense in the regular season and held Kevin Durant to just 39% shooting in their first-round sweep of the Brooklyn Nets. But Brooklyn actually scored more efficiently (115.0 points per 100 possessions) over those four games than it did in the regular season (113.2), and though it didn’t necessarily feel that way, the Celtics’ victory was more about the other end of the floor. The Celtics have had the second-most efficient offense in these playoffs, having scored 119.2 points per 100 possessions against Brooklyn. And while they had the league’s No. 1 defense over the full regular season season, they also had its top-ranked offense (120.7 per 100) as they won 26 of their final 32 games.

The regular-season series between these two teams was relatively offensive, with the Celtics and Bucks combining to score almost 116 points per 100 possessions over the four games. But the Celtics’ defense has generally played at a high level all season and the Bucks’ defense has seemingly flipped the switch.

— John Schuhmann

The pick

Fully healthy, these teams are evenly matched, having split the regular season series 2-2. But Middleton’s knee injury casts doubt on his availability for the early portion of this series. By the time he’s fully healthy, it just might be too late. The Celtics are close to peaking at just the right time with a hungry, battle-tested group of young players and a first-year head coach in Udoka who has ratcheted up the club’s collective tenacity on both ends of the floor. The defending champion Bucks showed a little vulnerability in the first round, losing to the Bulls at home in Game 2. This series should go down to the wire, but Middleton’s injury will be too much for Milwaukee to overcome. Celtics in 7.

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Michael C. Wright is a senior writer for NBA.com. You can e-mail him here, find his archive here and follow him on Twitter.

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