British light-heavyweight Buatsi earned a win yet underwhelming beginning to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a split decision in a tight ten-round contest at the Manchester venue.
But Parker had his moments, connecting with powerful individual punches in a match where the styles didn't complement each other, with constant grabbing disrupting the contest.
Two judges judged it 96-94 for to the winner, with the final judge a even score.
"It wasn't my greatest performance but what do champions do? They win," the victor stated in the ring as he was met by negative reactions from the crowd.
The British fighter returned to winning ways after his maiden career setback - an thrilling contest with Callum Smith in February - to earn a 20th pro success.
Some ringside observers believed Parker, 31, deserved the win but the Derbyshire fighter experienced the second defeat of his 28-bout professional journey.
The Champion Secures Triumph in Dull Encounter
His career development has stalled in recent times through promotional disputes and long spells of inactivity.
Previously predicted as the rising talent of the British fight scene, he now stands trying to achieve expectations - perhaps in what seems like a last chance - under recent support.
He began cautiously, with both competitors ready to be aggressive in a tentative initial rounds.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in 2022 after suffering a hand injury during the bout, was competing in only his second bout at light-heavyweight.
By the middle rounds, the underdog had stayed in contention, making life awkward for Buatsi, who utilized a tight guard to narrowly win the more effective rounds.
The spectators' acceptance wore thin amid regular grabbing, with Parker slipping to the canvas several times during clinches.
Parker ended stronger, but multiple periods were challenging to assess. Nevertheless, previous titleholder Carl Frampton - providing analysis for Dazn - had it significantly in favor for the challenger, awarding Buatsi just only one stanza.
The victor advances, but with the likes of heavy hitter David Benavidez, and Russian champion Dmitry Bivol ruling the division, the journey to titles - based on this performance - still looks steep.
Ricky Hatton Homage Inspires Local Rivalry Fight
In the secondary fight, Lyndon Arthur overcame Rea by close verdict in a outstanding Manchester vs Manchester battle to capture the continental title.
The stadium became quiet for a deeply poignant memorial before the fight as a bell ceremony sounded in honor of local hero Hatton, who succumbed 46 years old in last month.
It was the premier televised match card in the location since Hatton's death. As the final chime ended, the silence became a shout of "only one Ricky Hatton".
The competition within the ring reflected the sentiment. The eventual winner was floored by a left hook directly to the chin in the initial stage, getting up unsteadily to his feet on the count of eight.
But the red-colored shorts of Arthur soon came forward again. From the other corner, the opponent - in competitive blue - was marked up early, noticeable damage developing under his eye as the pair landed hard punches.
The challenger, twenty-seven, landed power punches but Arthur, 34, fought fire with fire with effective uppercuts as the action seldom decreased over 12 rounds.
Arthur edged the result with scoring tallies of 114-114, narrow lead and clearer margin, but the two athletes were relentless and courageous, everything their hero Hatton represented.
