UK light-heavyweight Buatsi made a winning yet underwhelming debut to life under new promoters with a unanimous decision over Zach Parker in Manchester.
The Olympic medal winner's punch output likely gained him a judges' verdict in a close ten-round fight at the Manchester venue.
But Parker had his moments, landing some telling individual punches in a fight where the styles didn't complement each other, with excessive holding disrupting the contest.
The scoring judges had it 96-94 for to Buatsi, with the final judge a even score.
"It wasn't my greatest performance but what defines champions? They win," the victor commented in the ring as he was met by negative reactions from the audience.
The British fighter returned to winning ways after his first professional defeat - an epic battle with Callum Smith in last winter - to secure a 20th pro success.
Various analysts believed Parker, 31, deserved the decision but the contender suffered the second setback of his 28-bout professional journey.
The Champion Earns Win in Slow Affair
His career development has plateaued in the past few years through management issues and long spells of inactivity.
Previously predicted as the next star of British boxing, he now is positioned trying to realize his potential - perhaps in what seems like a final roll of the dice - under fresh management.
He opened tentatively, with both competitors willing to overcommit in a tentative initial rounds.
Parker, whose sole prior loss came at the lower weight class against his former opponent in 2022 after suffering a hand injury during the bout, was competing in only his second appearance at the higher weight class.
By the middle rounds, Parker had made it competitive, making life awkward for Buatsi, who worked behind a tight guard to barely take the better periods.
The audience's tolerance decreased amid frequent holding, with the challenger going down various instances during close exchanges.
Parker closed well, but many rounds were challenging to assess. Although, ex-champion Carl Frampton - providing analysis for Dazn - had it particularly wide for the challenger, giving Buatsi just a single round.
The victor moves on, but with the caliber of heavy hitter David Benavidez, and Russian champion Bivol ruling the division, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Memorial Inspires Local Rivalry Contest
In the supporting bout, Lyndon Arthur defeated Rea by divided decision in a excellent all-Manchester contest to capture the continental belt.
The venue grew hushed for a profoundly moving tribute prior to the bout as a bell ceremony tolled in memory of Manchester's own Ricky Hatton, who died at age 46 in recently.
It was the first major televised boxing event in the city since Hatton's passing. As the last bell disappeared, the quiet became a chant of "only one Hatton".
The action during the bout equaled the sentiment. Arthur was sent to the canvas by a powerful punch squarely on the chin in the early round, getting up unsteadily to his footing on the referee's count.
But the Manchester United-red attire of Arthur soon advanced again. Across the ring, Rea - in rival City blue - was damaged soon, heavy swelling developing under his eye as the fighters exchanged hard punches.
The challenger, twenty-seven, delivered effective strikes but Arthur, thirty-four, fought fire with fire with effective uppercuts as the action seldom decreased over the full distance.
The victor edged the decision with scoring tallies of 114-114, narrow lead and 115-112, but each competitor were determined and full of heart, all the qualities their inspiration the legend represented.
