Working without two starters, Tom Thibodeau’s decision to turn to confident rookie Quentin Grimes instead of benched veteran Kemba Walker on Sunday turned out to be unarguable.

The absences of RJ Barrett (health and safety protocols) and Alec Burks (personal reasons) still left the skidding Knicks without enough to seriously challenge the defending NBA champions in a 112-97 matinee loss to the Bucks at the Garden.

“We fell short. Anytime you fall short there’s disappointment,” Thibodeau said. “I think anytime you’re shorthanded, you have to think about things.

“But our margin of error is real small right now.”

Obi Toppin also sat out due to the NBA’s COVID-19 protocols as the Knicks (12-15) lost for the sixth time in seven games with Steph Curry and the Warriors coming to town on Tuesday.

Thibodeau went with just an eight-man rotation until the closing seconds, with Grimes — the 25th-overall pick in the draft out of Houston — flourishing in his first NBA start with a career-best 27 points in 40 minutes, including a team rookie record seven 3-pointers (on 13 attempts).

According to the Elias Sports Bureau, it also marked the most points scored by a Knicks rookie in his first NBA start since starters began being tracked in 1970-71.

“I’m not going to lie, I really didn’t [expect it]. I found out about 90 minutes before the game … that I was going to start,” Grimes said. “I was just trying to get my mind right and trying to soak it all in.

“I felt like I learned a lot about myself going against the defending champs.”

All-Star forward Julius Randle endured yet another poor outing, matching his season low with just eight points on 2-for-9 shooting with a season-high tying seven turnovers and a minus-17 rating.

Thibodeau assessed that Randle “tried to make the right plays,” but opposing defenses “have loaded up on him pretty good” with double teams.

“Like, I can start forcing it, but what would that say about my trust in my guys?” said Randle, who did have 10 rebounds and seven assists. “So I just try to go out there and make the right plays. At least my intentions are to make the right plays. Just try to take what the game gives me.”

Little-used former lottery pick Kevin Knox also saw his first extended playing time of the season ahead of the benched Walker, netting 18 points with five 3s over 29 minutes. Despite the rotation openings, the Bronx-reared Walker sat out for the eighth consecutive game since his last appearance on Nov. 26.

Derrick Rose moved into the starting lineup in place of Burks and totaled 18 points with seven assists over 36 minutes, matching a season high for minutes.

Grimes, whose previous high in minutes was 17, played the entire first quarter and the first four minutes of the second to net 13 points — including three 3s — by halftime.

Still, the undermanned Knicks trailed by as many as 21 in the second quarter before late 3-pointers by Grimes and Evan Fournier drew them within 61-48 at intermission.

Grimes came out firing to open the third — while also firing up the home fans — by striking four times from downtown in the first 3:07 to help close the gap to nine points.

“It’s a feeling like no other, especially in MSG,” Grimes said. “That was my first time feeling the crowd like that. It was definitely something I will take in. But hopefully we can come out next time with a dub [W, for win], for sure.”

The Bucks (18-10) pushed the cushion back to 14 in the closing minute of the third on Grayson Allen’s 3 and a Giannis Antetokounmpo drive for a 93-79 lead entering the final period.

Khris Middleton led Milwaukee with 24 points, while Antetokounmpo recorded 20 with 11 assists and 10 rebounds, and former Knick Bobby Portis added 19 points.

“They’re the defending champs, so they’re tough,” Thibodeau said. “They got off to a good start. New starting lineup, we adjusted quickly. I thought the fight was better in the second half. … But if you have two or three bad minutes against them, they’re going to make you pay.”