The Boston Celtics and Toronto Raptors appeared to be the top challengers to the Milwaukee Bucks throughout the 2019-20 NBA regular season.

Toronto and Boston finished second and third in the East, respectively, and–despite a long hiatus–have each looked especially sharp in Orlando.

The Raptors went 7-1 during the seeding games before absolutely dismantling a makeshift Brooklyn Nets roster in the first round.

While the Celtics lost Gordon Hayward to an ankle injury in Game 1, they still managed to sweep past the Philadelphia 76ers with relative ease.

Now, the battle is really on.

For the Celtics, the questions are likely to revolve around Hayward's absence and whether the trio of Kemba Walker, Jayson Tatum and Jaylen Brown can provide enough consistent offense.

However, Boston's most important player will be lurking on the interior. Daniel Theis has essentially attempted to mimic what Al Horford brought to last year's squad, and he will be of vital importance against the Raptors.

Unlocking Kemba

Celtics fans understand Tatum and Brown have excelled as shot creators. They can make plays off the bounce and take opposing wings to the cup.

Walker can also create for himself, but he is at his best in pick-and-roll, when he can force opposing bigs to hedge and switch or seek an open jumper behind the screen.

Theis is Boston's most important on-ball screener. He can roll to the rim for easy two's or stay behind the three-point line and fire away from the perimeter. Theis can also make the extra pass to start ball movement.

It is not coincidental both Walker and Theis had their best games in the series finale against the Sixers.

Walker was relentless in attacking the rim, finishing the contest while also shooting 4-for-9 from beyond the arc. Meanwhile, Theis scored 15 points and made two of his five triples.

Theis' ability as a screener and simultaneous threat as a big opens up the paint and makes Walker that much dangerous as a slasher.

Speaking of opening up the paint.

Taking away Marc Gasol

Toronto's frontcourt is full of rim-protectors and deterrents, and Gasol anchors the Raptors' frontcourt defense.

Gasol is one of the more underrated paint defenders in basketball. His defense on Joel Embiid in last year's Eastern Conference semifinal was a major part of Toronto's championship narrative, and he is a big body who makes things difficult at the rim.

The Raptors pursue shooters and force the ball to go inside. Opponents averaged 38.9 threes per game against Toronto, which ranked 29th in the league. Meanwhile, the Raptors held opponents to 33.7 percent shooting from deep, the best mark in the league.

Most of this defensive strategy is the result of guards like Kyle Lowry and Fred VanVleet hounding the ball, with active wings such as OG Anunoby, Norman Powell and Pascal Siakam playing passing lanes and being physical. The Raptors can afford to take these risks knowing Gasol lurks around the basket.

But Theis' comfort playing on the perimeter could force Gasol away from the rim, which is huge for the Celtics.

Theis has shown more assertiveness on the offensive end. He averaged just 1.5 threes per game during the regular season, but that number jumped to nearly four per game against the Sixers.

Granted, Theis made just three of his 15 attempts from beyond the arc. It seems likely the Raptors will make him shoot the ball before seriously contesting.

But if Theis commits to spreading the floor and forces Gasol out onto the perimeter from time, he might open things up for Tatum, Brown and others to find driving lanes.

Celtics head coach Brad Stevens previously said the team needs Theis to be a “threat,” which holds even more weight without Hayward on the floor.

The German big man is Boston's X-factor heading into what should be one of the most tightly-contested matchups of the playoffs thus far.