UK light heavyweight Buatsi earned a win yet disappointing debut to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output seemingly secured him a split decision in a tight ten-round contest at the boxing arena.
But Parker landed significant shots, landing some telling single shots in a contest where the styles never quite clicked, with frequent clinching disrupting the action.
The scoring judges judged it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my finest showing but what defines champions? They prevail," Buatsi said in the arena as he was faced negative reactions from the audience.
The British fighter got back to winning after his initial pro loss - an epic battle with Callum Smith in last winter - to earn a twentieth professional win.
Several boxing experts felt Parker, thirty-one, merited the decision but the contender experienced the second setback of his 28-match career.
The Champion Claims Triumph in Uneventful Encounter
The fighter's advancement has plateaued in the past few years through management issues and long spells of inactivity.
Once tipped as the rising talent of British boxing, he now is positioned trying to achieve expectations - maybe in what appears to be a ultimate opportunity - under fresh management.
He began cautiously, with neither fighter prepared to overcommit in a tentative opening few rounds.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in 2022 after damaging his hand during the bout, was participating in his second appearance at light heavyweight.
By the halfway stage, Parker had kept it close, making life awkward for Buatsi, who operated with a protective stance to narrowly win the more effective periods.
The audience's tolerance decreased amid regular grabbing, with Parker slipping to the canvas several times during clinches.
Parker ended stronger, but several stanzas were hard to judge. However, ex-champion Carl Frampton - working with Dazn - had it particularly wide for the challenger, scoring for Buatsi just a single round.
Buatsi moves on, but with the presence of power puncher David Benavidez, and Russian champion Bivol dominating the weight class, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Tribute Fuels All-Manchester Battle
In the secondary fight, Arthur overcame Bradley Rea by close verdict in a terrific all-Manchester contest to capture the European light-heavyweight belt.
The stadium became quiet for a deeply poignant homage before the fight as a bell ceremony tolled in honor of the city's native Ricky Hatton, who succumbed at age 46 in September.
It was the first major televised fight night in the location since Hatton's passing. As the concluding sound disappeared, the quiet gave way to a shout of "only one Ricky Hatton".
The fighting within the ring reflected the sentiment. The eventual winner was floored by a left hook flush on the face in the initial stage, getting up unsteadily to his feet on the eight count.
But the Manchester United-red shorts of the fighter soon advanced again. From the other corner, the opponent - in rival City blue - was marked up quickly, noticeable damage forming under his face as the duo traded powerful shots.
Rea, 27, landed power punches but Arthur, thirty-four, responded in kind with effective uppercuts as the intensity rarely dropped over twelve rounds.
Arthur edged the decision with judges scorecards of even score, 115-113 and 115-112, but both fighters were relentless and brave, everything their hero Hatton represented.
