Boston’s Jayson Tatum, right, Milwaukee’s Giannis Antetokounmpo will both be key offensive performers in the second-round series which begins on Sunday. Jon Durr/Associated Press

Boston and Milwaukee split four games in the regular season, but so much has changed leading into what should be the most-anticipated second round series in the NBA playoffs.

The Celtics and Bucks will begin their second-round showdown with Game 1 on Sunday afternoon. The defending champion Bucks will be without one key piece of – Khris Middleton – who sprained the medial collateral ligament in his left knee. Middleton said Friday he hoped to return in two weeks, a timeline that would put him back in the lineup for Game 3 on May 7 in Milwaukee.

Let’s take a closer look at five questions to consider as the Celtics attempt to take down the defending champion Bucks.

Do the Bucks keep starting big?

Milwaukee has had to adapt on the fly in the postseason after Middleton went down in Game 2 of the Bulls series. Without him, Coach Mike Budenholtzer made an interesting pivot, electing to go with a jumbo frontcourt with Bobby Portis in place of Middleton alongside Giannis Antetokounmpo and Brook Lopez. That new look group led the Bucks to three straight easy wins to help close out the series.

Many might say not mess with something that’s working in the postseason, but the Bucks face a Boston starting five that comes equipped with two 25-point wing scorers – Jayson Tatum and Jaylen Brown – and a point guard in Marcus Smart who has helped set the table for an elite Celtics offense over the past few months. The Bucks playing big up front would force either Portis or Giannis to guard one of Boston’s top scorers or Smart, something that could leave them exposed defensively.

Milwaukee could also insert a wing into the lineup such as Grayson Allen or Pat Connaughton. Both are key parts of the team’s second unit and would be giving up length to Tatum or Brown. Trying to hide one on Smart is an option, but there’s really not an appealing option here for the Bucks with Middleton out of the fold. Going smaller though seems to make the most sense for them defensively.

How much can the Celtics expect from Robert Williams III?

The Celtics usual starting center played a total of 24 minutes in his two games back since returning from knee surgery. A week of additional recovery time should help him get into better shape, which was a concern for Coach Ime Udoka.

“Physically, he is fine,” Udoka said. “The wind I don’t think was quite there those last two games. … Just get some live work in the last few days and just get him built back up so he can get back to his normal rotation, lineups, and minutes.”

The Celtics obviously will need to rely on much more on Robert Williams III in this series given the size the Bucks have on their front line. If Williams is still looking rusty or not close to 100 percent, will Udoka opt for an offense-first look from the perimeter and lean more on Grant Williams? Or will the size and rebounding Robert Williams can provide be worth the potential drawbacks? The Bucks also have shooters at all five positions on the floor so Williams’ mobility will be tested defensively, which could give Udoka a quick leash.

Milwaukee will be without forward Khris Middleton, left, for at least two games, while Boston is still working Rob Williams back into the lineup after he returned from a knee injury. Jon Durr/Associated Press

Who picks up the scoring load with Middleton out?

Defending the Bucks will be far easier without worrying about the All-Star wing who has routinely feasted on Boston’s defenders in the postseason in previous years.

Without him, the scoring onus will fall to other secondary players that the Celtics should match up well against. Jrue Holiday will become a primary creator when Giannis doesn’t have the ball and bigs like Lopez and Portis will need to get aggressive with their offense, something that will not come easy against the Celtics size on the front line. The X-factor for Udoka may come down to how he decides to defend Bucks bench weapons like Allen.

If the Celtics frontcourt can hold its own against Antetokounmpo (with help), Milwaukee’s scoring options could dry up quickly if guys like Holiday and Allen are corralled. Ultimately, the Bucks will need a hot 3-point shooting series to be able to stay in this matchup. They do have a far better system than Brooklyn that will test the Celtics but there’s not a lot of scary guys outside of Giannis.

How do the Celtics elect to defend Giannis?

Udoka has more options on this front than perhaps any team in the NBA. Al Horford and Robert Williams will likely get the main assignments, but Brown and Grant Williams will get their chance as well. Antetokounmpo has not shot well from the perimeter all year long (30 percent in regular season and 18 percent in playoffs from 3-point range) so that could be the choice easier for Udoka. Simply keeping bigger size on him and daring him to beat Boston from the perimeter is a winning play from a percentages standpoint.

The question will be how hard does Udoka focus on making sure Antetokounmpo doesn’t get going inside the paint. Throwing constant double teams at him opens the door for the Bucks shooting to get going. There’s a case to be made for letting Antetokounmpo get his in 1-on-1 setting and shutting down open looks for everyone else. A mix of both is likely what we will see.

Can Celtics make Bucks pay for their willingness to give up 3-pointers?

This will be the biggest question of the series from a Celtics standpoint. The Bucks have been built on protecting the paint under Budenholtzer and surrender more 3s than any team in the NBA this year.

The Celtics bounced back from a miserable shooting start to the year to finish 14th overall in 3-point percentage. They shot average as a team against the Nets (35 percent) but Udoka will have the ability to play with shooting at all five positions on the floor if he wants with the emergence of Payton Pritchard and Grant Williams from beyond the arc. Those guys will be giving up some size in certain matchups but the offensive tradeoff could be well worth it if the Bucks are daring the Celtics to beat them from long range.