UK light-heavyweight Buatsi earned a win yet unimpressive debut to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a majority decision in a tight ten-round contest at the Manchester venue.
But Parker found success, landing some telling individual punches in a contest where the styles failed to mesh, with frequent clinching disrupting the action.
The officials scored it 96-94 for to Buatsi, with the third a even score.
"It wasn't my greatest showing but how do champions respond? They win," the victor stated in the ringside as he was faced disapproval from the crowd.
The British fighter bounced back after his first professional defeat - an epic battle with Callum Smith in last winter - to secure a 20th pro success.
Various analysts thought Parker, 31, merited the decision but the contender endured the second defeat of his 28-fight professional journey.
The Champion Secures Victory in Dull Match
The fighter's advancement has stalled in the past few years through management issues and long spells of lack of fights.
Once tipped as the rising talent of British boxing, he now is positioned trying to fulfil that early promise - perhaps in what seems like a last chance - under recent support.
He opened tentatively, with both competitors ready to be aggressive in a calculated early stages.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in 2022 after breaking his hand in the contest, was competing in only his second bout at light heavyweight.
By the middle rounds, the underdog had stayed in contention, causing problems for Buatsi, who utilized a tight guard to narrowly win the cleaner rounds.
The crowd's patience wore thin because of constant clinching, with the challenger falling to the floor multiple occasions during grappling.
Parker ended stronger, but multiple periods were difficult to score. Nevertheless, former world champion Carl Frampton - commentating on Dazn - had it significantly in favor for Parker, awarding Buatsi just one round.
The victor progresses, but with the caliber of heavy hitter Benavidez, and Russian star Bivol leading the category, the path to championship success - on this evidence - still looks steep.
Hatton Tribute Inspires All-Manchester Battle
In the secondary fight, Lyndon Arthur beat Bradley Rea by divided decision in a excellent local rivalry contest to win the European light heavyweight championship.
The stadium grew hushed for a deeply poignant homage prior to the bout as 10 bells tolled in remembrance of Manchester's own Hatton, who succumbed 46 years old in September.
It was the initial significant broadcast fight night in the city since Hatton's death. As the concluding sound ended, the stillness transitioned to a shout of "there's only one Hatton".
The fighting within the ring reflected the feeling. The eventual winner was knocked down by a significant blow squarely on the face in the second round, rising shakily to his position on the referee's count.
But the Manchester United-red equipment of Arthur soon pressed ahead again. Across the ring, the opponent - in competitive blue - was damaged early, heavy swelling developing under his eye as the fighters traded punishing blows.
Rea, 27, connected with strong blows but the champion, thirty-four, matched the intensity with stinging uppercuts as the action seldom decreased over the full distance.
Arthur narrowly won the decision with judges scorecards of 114-114, narrow lead and 115-112, but both fighters were relentless and full of heart, everything their hero the legend represented.
