UK light heavyweight Buatsi made a winning yet underwhelming debut to his career under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a split decision in a tight ten-round bout at the Manchester venue.
But Parker found success, connecting with powerful individual punches in a contest where the styles failed to mesh, with constant grabbing disrupting the fight.
Two judges judged it 96-94 for to the winner, with the remaining official a even score.
"It wasn't my greatest display but what defines champions? They win," the victor said in the arena as he was met by disapproval from the spectators.
The British fighter got back to winning after his maiden career setback - an memorable fight with Callum Smith in February - to claim a twentieth professional win.
Some ringside observers thought Parker, thirty-one, deserved the victory but the Derbyshire fighter suffered the second loss of his 28-match career.
Buatsi Claims Win in Dull Encounter
Buatsi's progress has plateaued in recent years through promotional disputes and long spells of lack of fights.
Once tipped as the future of British boxing, he now is positioned trying to achieve expectations - possibly in what appears to be a last chance - under recent support.
He began cautiously, with neither fighter willing to be aggressive in a tentative opening few rounds.
Parker, whose only previous defeat came at super middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was competing in only his second appearance at light-heavyweight.
By the halfway stage, Parker had made it competitive, causing problems for the favorite, who operated with a protective stance to narrowly win the cleaner stanzas.
The crowd's patience decreased because of constant clinching, with Parker going down multiple occasions during close exchanges.
The underdog finished the stronger, but multiple periods were difficult to score. However, ex-champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just a single round.
Buatsi progresses, but with the caliber of power puncher David Benavidez, and Russian star Bivol dominating the weight class, the path to championship success - judging by this showing - still looks steep.
Ricky Hatton Memorial Fuels Manchester Derby Contest
In the supporting bout, Arthur overcame Bradley Rea by split-decision in a outstanding Manchester vs Manchester battle to secure the European light heavyweight title.
The stadium grew hushed for a extremely emotional memorial ahead of the contest as ten bells rang in memory of Manchester's own Hatton, who passed away 46 years old in September.
It was the premier televised boxing event in the city since the passing of Hatton. As the final chime faded, the stillness transitioned to a shout of "only one Hatton".
The action inside the ropes matched the sentiment. Arthur was sent to the canvas by a significant blow directly to the face in the early round, getting up unsteadily to his position on the count of eight.
But the Manchester United-red attire of the fighter soon came forward again. From the other corner, the opponent - in rival City blue - was injured quickly, noticeable damage appearing under his face as the duo landed hard punches.
The challenger, twenty-seven, connected with strong blows but the champion, 34, fought fire with fire with effective uppercuts as the pace rarely dropped over the full distance.
The victor edged the verdict with scoring tallies of 114-114, 115-113 and clearer margin, but the two athletes were persistent and full of heart, everything their idol Hatton represented.
