British light heavyweight Buatsi secured a victory yet disappointing start to his career under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a judges' verdict in a close 10-round contest at the boxing arena.
But Parker landed significant shots, landing some telling clean blows in a fight where the styles didn't complement each other, with excessive holding spoiling the action.
The scoring judges judged it 96-94 to Buatsi, with the third a even score.
"It wasn't my best display but how do champions respond? They win," Buatsi said in the ring as he was met by negative reactions from the spectators.
The Londoner bounced back after his maiden career setback - an epic battle with Callum Smith in last winter - to secure a twentieth professional win.
Various analysts thought Parker, 31, should have been awarded the win but the contender suffered the second defeat of his 28-fight professional journey.
Buatsi Secures Triumph in Dull Affair
The fighter's advancement has stalled in the past few years through contract problems and long spells of inactivity.
Once tipped as the rising talent of British boxing, he now finds himself trying to achieve expectations - possibly in what seems like a last chance - under fresh management.
He began cautiously, with both competitors willing to overcommit in a calculated initial rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in two years ago after damaging his hand in the contest, was competing in only his second fight at light-heavyweight.
By the middle rounds, the underdog had kept it close, causing problems for Buatsi, who operated with a protective stance to narrowly win the more effective periods.
The crowd's patience decreased during regular grabbing, with the challenger falling to the floor multiple occasions during close exchanges.
The underdog closed well, but many rounds were hard to judge. However, former world champion Carl Frampton - working with the broadcast network - had it significantly in favor for Parker, giving Buatsi just only one stanza.
The victor advances, but with the caliber of power puncher David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still appears challenging.
Ricky Hatton Tribute Fuels Manchester Derby Fight
In the secondary fight, Arthur defeated Bradley Rea by divided decision in a outstanding Manchester vs Manchester battle to win the European light heavyweight championship.
The stadium fell silent for a profoundly moving memorial before the fight as ten bells rang in memory of the city's native Ricky Hatton, who died at age 46 in September.
It was the first major televised match card in the location since the passing of Hatton. As the concluding sound ended, the silence transitioned to a roar of "there's only one Hatton".
The action inside the ropes reflected the feeling. Arthur was knocked down by a powerful punch directly to the chin in the early round, rising shakily to his footing on the referee's count.
But the traditional red equipment of Arthur soon advanced again. On the opposite side, Rea - in competitive blue - was marked up quickly, noticeable damage forming under his eye as the duo exchanged hard punches.
Rea, twenty-seven, delivered effective strikes but Arthur, thirty-four, matched the intensity with effective uppercuts as the action rarely dropped over 12 rounds.
The victor barely secured the verdict with judges scorecards of 114-114, narrow lead and clearer margin, but each competitor were determined and courageous, all the qualities their idol the legend stood for.
