The Boston Celtics (43-22) were already facing an uphill battle Tuesday night in San Antonio.

They were without key rotation players, still working Jayson Tatum back into rhythm after his long injury absence, and dealing with a tough road environment against one of the league’s best teams.

When Jaylen Brown was ejected midway through the second quarter, the margin for error effectively disappeared.

To their credit, Boston never stopped fighting.

The Celtics stayed competitive deep into the fourth quarter behind strong performances from Derrick White and several unlikely contributors off the bench.

But ultimately, the San Antonio Spurs (48-17) had too much firepower, pulling away late for a 125-116 win at the Frost Bank Center.

With the win, the Spurs completed a regular season sweep of the Celtics.

…emphasis on regular season, but we’ll cross that bridge when we get there.

Here are four takeaways from Tuesday’s loss:

Brown was putting together a strong performance before his night ended abruptly.

The Celtics star picked up two technical fouls within seconds of each other with 3:38 remaining in the second quarter after arguing a call with the officiating crew.

The second technical, issued by an official away from the initial exchange, triggered an automatic ejection.

Brown had 8 points and 7 assists in just 15 minutes before leaving the floor, clearly frustrated as teammates and team staff worked to guide him toward the locker room.

Already playing without Payton Pritchard and Nikola Vucevic, Boston suddenly found itself missing another major piece of its offense.

Every game still carries a bit of a “return to normal” feel for Tatum.

Tuesday marked just his third appearance since returning from the Achilles injury that sidelined him for much of the season.

The Celtics star finished with 24 points and 5 rebounds while playing 27 minutes, matching the workload he handled in his previous two games.

There were flashes of the familiar version of Tatum - including a stretch of seven straight points in the fourth quarter - but there were also signs that the rhythm is still returning.

He finished 10 for 24 from the field and 4 for 14 from three.

For Boston, the process of ramping him up remains the priority.

With the Celtics short-handed, Ron Harper Jr. stepped into a much larger role and delivered the best performance of his NBA career.

Harper poured in a career-high 22 points on 8 of 11 shooting, knocking down six of his nine attempts from three-point range.

He also enjoyed a unique moment during the game, matching up at times against his younger brother, Dylan Harper - which went the opposite of how it went for him at All-Star Weekend when guarding baby bro.

For a player who has bounced in and out of the rotation this season, Tuesday was a reminder of the scoring punch he can provide when given extended minutes.

Ultimately, the night belonged to Victor Wembanyama.

The Spurs superstar dominated with 39 points and 11 rebounds while shooting 8 of 15 from three-point range, repeatedly stretching Boston’s defense beyond its comfort zone.

De'Aaron Fox added 25 points and 9 assists as San Antonio pulled away late.

If these teams meet again later this year, the stakes will be much higher.

For now, the Celtics will turn their attention to the final stop of their road trip - a Thursday night showdown with the defending champion Oklahoma City Thunder (51-15).

Remember to join our CELTICS on ROUNDTABLE community, which is FREE! You can post your own thoughts, in text or video form, and you can engage with our Roundtable staff, as well as other Celtics fans. If prompted to download the Roundtable APP, that's free too!

Tom Carroll is a contributor for Roundtable, with boots-on-the-ground coverage of all things Boston sports. He's a senior digital content producer for WEEI.com, and a native of Lincoln, RI.