BOSTON -- If things went according to plan for the Boston Celtics, young players such as Jaylen Brown and rookie Jayson Tatum would have been asked to continue their development on a team that had a new Big Three.

However, when Gordon Hayward went down less than six minutes into the first game, and with Marcus Morris and then Marcus Smart also getting hurt, things changed.

On Tuesday night, Brown and Tatum did exactly what their coach hoped -- and expected -- they would do.

"I'm sure I'll say it down the road or tomorrow, or whenever I'm asked ... they are young guys, but for us to be in the situation that we're in, they're (just) guys," Celtics coach Brad Stevens said after Brown and Tatum combined for 45 points in a 110-89 rout of the winless New York Knicks.

"We need them to be guys, so ... I was just asked and I'll deal in reality and let everyone else praise them and we need them to play well on Thursday (at Milwaukee). We're in a situation right now where we're going to expect a lot out of those guys, we need them to be great, we need them to respond to adversity, we need them to respond to pats on the back just the same.

"We have high expectations for them, they should have high expectations for themselves -- and they're getting a great opportunity."

Brown, the third overall pick in the 2016 NBA Draft, scored 23 points. Tatum, the No. 3 selection in the 2017 draft (after the Celtics traded down), had 22 points, the pair combining to shoot 7 of 12 from 3-point range while playing hounding defense. Tatum, who joined Lonzo Ball as teenagers who have already scored 20 points in a game, collected four steals.

Tatum and Brown became the first 21-or-under duo in club history to both score 20 in a game. Brown did it on his 21st birthday.

"There's not too many people that can really stay in front of him ... He's exciting to watch out there," Tatum said of Brown.

Asked if he is feeling more assertive, Brown said, "Absolutely. It's a lot just guarding players then also turning it into offense, but it just takes a high level of being in shape and a mentality to just keep going forward and think, 'I got that.'"

Kyrie Irving had 20 points and seven assists, Al Horford added 13 points and 13 rebounds, and Daniel Theis scored 11 points for the Celtics (2-2). Boston earned its second consecutive win.

Horford also played much of the stellar defense that threw a blanket over New York's Kristaps Porzingis.

Porzingis, who was attempting to become the first Knick to scored 30 points in each of the first three games, was 1 of 12 from the floor before hitting two fourth-quarter baskets. Horford did much of the heavy physical work against Porzingis, and it seemed everyone on the floor for Boston took a shot at him at one time or another.

After scoring 64 points in the first two games, Porzingis left with 4:44 remaining. He finished with 12 points and five rebounds.

"There's a lot of really good defenders (in the NBA), but Al's up there," Stevens said.

The Celtics went 14 of 29 from 3-point range and held the Knicks without a 3-pointer until the fourth quarter. New York finished 1 of 12 from behind the arc.

Enes Kanter, the only Knick to play well, had 16 points and 19 rebounds in 30 minutes for New York (0-3). Former Celtic Courtney Lee added 13 points and Willy Hernangomez 12 (in 11 second-half minutes) in the loss.

"(We) couldn't get anything offensively," said New York's Tim Hardaway Jr., who was 2 of 11 from the floor, including 0 of 5 on 3-pointers, scoring six points. "We were all out there running around like we don't know what's going on, and it can't happen.

"It's frustrating, but at the same time it's early in the season so we just got to go back to the drawing board and get back to practice and just get out there knowing what we have to do offensively and defensively."

NOTES: With F Gordon Hayward gone, probably for the year (ankle, leg), the Celtics were also still without G Marcus Smart (ankles) and F Marcus Morris (knee). Smart missed his second straight game but is going on the upcoming two-game road trip, while Morris is targeting Oct. 30 for his Celtics debut. ... Patrick Ewing and Willie Naulls were the only other Knicks to open a season with two 30-point games before F Kristaps Porzingis did it this season. ... The Knicks host the Brooklyn Nets on Friday night, while the Celtics are at Milwaukee on Thursday and Miami on Saturday. ... G Courtney Lee's 3-pointer in the fourth quarter ended any chance of the Knicks failing to hit a trey in game for the first time since March 6, 2010. The Celtics last held a team without a 3-pointer March 7, 2012.