The Celtics were the healthiest they’ve been all season Sunday in Oklahoma City, but they didn’t escape their loss to the Thunder unscathed.

Jayson Tatum had an unfortunate run-in with the front row of the Paycom Center seats when a third-quarter collision with Thunder forward Jalen Williams sent him tumbling out of bounds. Both players were chasing after a loose ball after Tatum deflected a cross-court pass by Jaylin Williams.

Tatum was slow to get up and received attention from members of Boston’s medical staff, but he remained in the game. He played 39 minutes and led the Celtics in points (26) and rebounds (10) in a 105-92 loss to the Western Conference-leading Thunder.

“My arm’s just bruised,” Tatum told reporters after the game. “I kind of went forearm-first into the chair. It’s bruised, scraped all up, but I’ll be all right.”

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Tatum was one of Boston’s only viable scoring options during a dismal second half. The Thunder, who boast the NBA’s top-ranked defense, held the Celtics to 27 second-half points, and Jaylen Brown, Derrick White, Jrue Holiday, Payton Pritchard and Sam Hauser all were held scoreless after halftime, going a combined 0-for-19.

As a team, the Celtics shot 8-for-40 from the floor (20%) and 3-for-24 from 3-point range (12.5%) in the second half as OKC turned a 10-point halftime deficit into a 13-point victory. It was Boston’s first loss by more than nine points this season.

Tatum, Brown — who scored 21 points before his second-half oh-fer — and head coach Joe Mazzulla all blamed poor spacing for the Celtics’ swoon.

“They may have picked up the pressure at points (in the second half), but our spacing allowed — not allowed them, but just played into it,” Tatum told reporters. “We’ve got to be aware of that in those moments and just be better in our spacing.”

The Celtics will finish up their four-game Western Conference road trip Tuesday night against the Denver Nuggets.