UK light heavyweight Buatsi secured a victory yet disappointing debut to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a split decision in a close 10-round contest at the Manchester venue.
But Parker landed significant shots, landing some telling individual punches in a match where the styles failed to mesh, with excessive holding spoiling the contest.
The scoring judges scored it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my best display but how do champions respond? They prevail," the victor said in the arena as he was met by boos from the crowd.
The British fighter returned to winning ways after his initial pro loss - an epic battle with Callum Smith in February - to secure a twentieth professional win.
Several boxing experts believed Parker, thirty-one, should have been awarded the win but the contender experienced the second defeat of his 28-bout career.
Buatsi Claims Win in Dull Encounter
The fighter's advancement has stalled in recent times through contract problems and long spells of lack of fights.
Earlier forecast as the rising talent of British boxing, he now is positioned trying to fulfil that early promise - possibly in what seems like a final roll of the dice - under new promotional backing.
He started carefully, with both competitors prepared to be aggressive in a tentative initial rounds.
Parker, whose single earlier setback came at super middleweight against John Ryder in 2022 after damaging his hand during the bout, was competing in only his second appearance at light-heavyweight.
By the middle rounds, the underdog had stayed in contention, making life awkward for the favorite, who operated with a protective stance to barely take the cleaner periods.
The spectators' acceptance decreased amid frequent holding, with Parker going down multiple occasions during close exchanges.
The underdog ended stronger, but multiple periods were hard to judge. Nevertheless, ex-champion Carl Frampton - working with the broadcast network - had it particularly wide for the challenger, giving Buatsi just only one stanza.
The victor progresses, but with the presence of heavy hitter Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the journey to titles - judging by this showing - still looks steep.
Ricky Hatton Homage Fuels Local Rivalry Contest
In the secondary fight, Arthur defeated Bradley Rea by close verdict in a outstanding all-Manchester fight to capture the continental championship.
The venue became quiet for a profoundly moving homage before the fight as a bell ceremony tolled in memory of the city's native Hatton, who succumbed aged 46 in recently.
It was the first major televised match card in the urban center since Hatton's passing. As the final chime disappeared, the quiet gave way to a roar of "there's just one Ricky Hatton".
The fighting inside the ropes matched the emotion. Arthur was sent to the canvas by a powerful punch directly to the chin in the second round, standing uncertainly to his position on the referee's count.
But the red-colored equipment of the fighter soon advanced again. On the opposite side, the opponent - in competitive blue - was injured early, noticeable damage developing under his orbital area as the fighters exchanged hard punches.
Rea, 27, delivered effective strikes but the champion, 34, matched the intensity with stinging uppercuts as the pace seldom decreased over twelve rounds.
The victor narrowly won the decision with scoring tallies of even score, close margin and clearer margin, but the two athletes were determined and brave, exactly what their idol the legend embodied.
