UK light heavyweight Buatsi secured a victory yet underwhelming debut to his career under recent management with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output likely gained him a split decision in a tight 10-round contest at the boxing arena.
But Parker found success, connecting with powerful single shots in a match where the techniques didn't complement each other, with frequent clinching marring the action.
The officials scored it 96-94 to Buatsi, with the remaining official a even score.
"It wasn't my greatest display but what do champions do? They win," Buatsi commented in the arena as he was met by disapproval from the spectators.
The British fighter bounced back after his initial pro loss - an epic battle with Callum Smith in February - to secure a 20th pro success.
Various analysts felt Parker, 31, merited the victory but the contender suffered the second defeat of his 28-fight career.
Buatsi Secures Victory in Dull Affair
Buatsi's progress has plateaued in the past few years through promotional disputes and long spells of lack of fights.
Once tipped as the next star of the British fight scene, he now stands trying to fulfil that early promise - possibly in what feels like a final roll of the dice - under new promotional backing.
He started carefully, with the two boxers willing to be aggressive in a cagey opening few rounds.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in two years ago after damaging his hand during the bout, was competing in only his second bout at light heavyweight.
By the halfway stage, Parker had kept it close, making life awkward for the favorite, who operated with a protective stance to edge the better stanzas.
The audience's tolerance diminished amid frequent holding, with Parker slipping to the canvas several times during clinches.
The underdog finished the stronger, but many rounds were hard to judge. Nevertheless, ex-champion Carl Frampton - commentating on Dazn - had it significantly in favor for Parker, scoring for Buatsi just one round.
Buatsi moves on, but with the caliber of heavy hitter David Benavidez, and Russian star Bivol ruling the division, the road to world glory - on this evidence - still looks steep.
Ricky Hatton Homage Inspires Local Rivalry Fight
In the secondary fight, Arthur overcame Bradley Rea by close verdict in a outstanding local rivalry contest to capture the European light heavyweight championship.
The arena became quiet for a extremely emotional homage ahead of the contest as ten bells tolled in remembrance of Manchester's own Hatton, who died at age 46 in recently.
It was the first major televised fight night in the city since Hatton's passing. As the last bell disappeared, the quiet became a shout of "there's only one Ricky Hatton".
The competition inside the ropes equaled the feeling. The eventual winner was knocked down by a significant blow directly to the chin in the early round, standing uncertainly to his footing on the referee's count.
But the red-colored shorts of Arthur soon pressed ahead again. On the opposite side, Rea - in rival City blue - was damaged early, significant bruising forming under his eye as the pair landed punishing blows.
Rea, twenty-seven, landed power punches but the champion, thirty-four, responded in kind with powerful upward punches as the intensity rarely dropped over 12 rounds.
Arthur edged the decision with scoring tallies of draw, 115-113 and 115-112, but each competitor were persistent and full of heart, exactly what their inspiration Hatton represented.
