UK light heavyweight Buatsi earned a win yet disappointing start to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a judges' verdict in a close ten-round bout at the Manchester venue.
But Parker landed significant shots, connecting with powerful clean blows in a match where the styles never quite clicked, with frequent clinching marring the contest.
Two judges judged it 96-94 to the winner, with the remaining official a even score.
"It wasn't my greatest display but what do champions do? They win," the victor stated in the arena as he was greeted with disapproval from the audience.
The Londoner got back to winning after his first professional defeat - an epic battle with his previous opponent in last winter - to secure a twentieth professional win.
Several boxing experts felt Parker, 31, should have been awarded the decision but the contender experienced the second defeat of his 28-match professional journey.
Buatsi Claims Triumph in Slow Match
Buatsi's progress has plateaued in the past few years through management issues and considerable time of inactivity.
Previously predicted as the future of UK boxing, he now is positioned trying to fulfil that early promise - perhaps in what feels like a final roll of the dice - under recent support.
He began cautiously, with the two boxers willing to be aggressive in a cagey initial rounds.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in 2022 after suffering a hand injury in the contest, was participating in his second fight at light heavyweight.
By the middle rounds, the underdog had stayed in contention, making life awkward for Buatsi, who worked behind a tight guard to barely take the more effective stanzas.
The spectators' acceptance decreased during constant clinching, with the challenger slipping to the canvas various instances during clinches.
The underdog ended stronger, but multiple periods were challenging to assess. However, ex-champion Carl Frampton - working with the broadcast network - had it significantly in favor for Parker, awarding Buatsi just one round.
The victor advances, but with the presence of hard hitter Benavidez, and Eastern European fighter Bivol dominating the weight class, the journey to titles - based on this performance - still appears challenging.
Ricky Hatton Memorial Motivates Local Rivalry Battle
In the secondary fight, Lyndon Arthur defeated Rea by divided decision in a outstanding local rivalry fight to secure the European light-heavyweight championship.
The venue became quiet for a deeply poignant memorial before the fight as 10 bells tolled in remembrance of Manchester's own Ricky Hatton, who passed away at age 46 in last month.
It was the initial significant broadcast match card in the location since Hatton's passing. As the concluding sound ended, the stillness gave way to a shout of "there's just one Hatton".
The fighting inside the ropes matched the sentiment. Arthur was sent to the canvas by a significant blow squarely on the face in the second round, rising shakily to his feet on the referee's count.
But the traditional red shorts of the fighter soon pressed ahead again. From the other corner, the opponent - in opposing blue - was marked up quickly, noticeable damage developing under his eye as the duo exchanged hard punches.
Rea, 27, delivered effective strikes but the champion, thirty-four, responded in kind with effective uppercuts as the action seldom decreased over the full distance.
Arthur narrowly won the result with judges scorecards of even score, 115-113 and wider gap, but both fighters were persistent and courageous, all the qualities their idol Hatton embodied.
