The Celtics still are the defending NBA champions. But it’s hard to call them the favorites to win it all again this year after what transpired Sunday in Oklahoma City.

Boston, at full strength for the first time all season, crumbled in the second half of a potential NBA Finals preview, losing 105-92 to Shai Gilgeous-Alexander and the Western Conference-leading Thunder.

The Celtics led by 10 at halftime but scored just 27 points over the final two quarters against an OKC squad that boasts the NBA’s top-ranked defense (and, now, a 15-game win streak). Gilgeous-Alexander, the early favorite for NBA MVP, was superb for the Thunder, finishing with 33 points, 11 rebounds, six assists, three steals and two blocks.

Jayson Tatum tallied 26 points and 10 rebounds to lead the Celtics, who shot a season-worst 19.6% from 3-point range (9-for-46). All 21 of Jaylen Brown’s points came in the first half.

Boston’s guard trio of Derrick White, Jrue Holiday and Payton Pritchard, who came in shooting a collective 54.8% from three over the previous three games, all were held scoreless after halftime. Pritchard finished without a point for just the second time this season.

The loss dropped the Celtics to 26-10 on the season and 2-1 on their current Western Conference road trip, which began with impressive back-to-back wins over Minnesota and Houston. They’ll close out that trip Tuesday night in Denver.

The big names on both sides delivered early, with Tatum, Brown and Gilgeous-Alexander scoring nine, 10 and 11 points in the first quarter, respectively.

Gilgeous-Alexander attacked the basket effectively with successful layups on his first three drives. That’s his bread and butter offensively; the Thunder star is averaging nearly three more points per game on drives than any other NBA player this season.

Driving also was an effective strategy for Brown, who did not appear hindered by the shoulder injury that kept him out of the previous two games, and Tatum. They went a combined 7-for-7 from the foul line in the first, with each drawing one foul by Gilgeous-Alexander.

With Brown returning, the Celtics had all of their players available for just the second time all season (excluding those on G League assignments). It was the first time their entire rotation was available for a full game and their seventh time fielding their preferred starting five.

OKC was down three contributors in Alex Caruso, Chet Holmgren and Ajay Mitchell, who missed the game with injuries.

The Celtics got a second-chance 3-pointer from Al Horford and a transition three from Sam Hauser late in the first quarter, then controlled play for most of the second, stretching their lead to double digits.

Brown maintained his aggressive offensive mindset, pouring in 21 first-half points while attempting just three shots outside the paint. As a team, Boston outscored Oklahoma City 24-4 in the paint in the second quarter, going 6-for-7 on shots in the restricted area while the Thunder went 1-for-5.

As time wound down in the first half, Tatum connected with Kristaps Porzingis on a fast-break alley-oop, then slammed home a putback dunk on the next possession to put the Celtics up 13. They led by 10 at the half, 65-55, the second-largest halftime deficit of the season for OKC.

That momentum didn’t last. The third quarter was a low-scoring, physical slugfest, with the Thunder outscoring the Celtics 21-15. Boston made just five field goals in the quarter, including a contested midrange jumper by Porzingis as time expired after OKC had cut its lead to one.

The third also featured a scary moment involving Tatum, who was slow to get up after falling headlong into the crowd while battling Lu Dort for a loose ball. Members of the Celtics training staff checked on Tatum, but he remained in the game.

But that didn’t help the Celtics during a disastrous fourth quarter, during which the Thunder turned a playoff-esque nailbiter into a runaway rout.

The pivotal stretch began when Dort stole a Jrue Holiday pass with 6:31 remaining and scored at the other end to put OKC up 89-85. Gilgeous-Alexander hit a three on the next Thunder possession; Porzingis was whistled for an offensive foul, his fifth; Gilgeous-Alexander blocked a fast-break Tatum layup; and Isaiah Hartenstein threw down a seismic dunk over Derrick White to cap a 9-0 run for the West front-runners.

A three by Tatum got the Celtics back to within six, but subsequent triples by Dort, nestled around a Gilgeous-Alexander steal, shut the door on Boston’s comeback bid.

The Celtics will conclude their road trip Tuesday against another MVP candidate: three-time winner Nikola Jokic, whose Nuggets beat Boston twice last season. Jokic is arguably the NBA’s most dangerous offensive player, and he’s put up staggering numbers of late, averaging 36.5 points, 16.5 rebounds, 11.3 assists and 2.0 steals per game over his last four contests.