UK light-heavyweight Buatsi secured a victory yet disappointing debut to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output seemingly secured him a split decision in a competitive 10-round contest at the boxing arena.
But the challenger landed significant shots, delivering effective clean blows in a contest where the styles didn't complement each other, with excessive holding marring the contest.
The scoring judges scored it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my finest performance but what do champions do? They prevail," the victor said in the ring as he was faced disapproval from the spectators.
The Londoner got back to winning after his maiden career setback - an thrilling contest with his previous opponent in last winter - to claim a 20th career victory.
Several boxing experts thought Parker, thirty-one, merited the win but the British boxer endured the second defeat of his 28-fight career.
Buatsi Secures Win in Slow Match
The fighter's advancement has slowed in recent times through contract problems and extended periods of lack of fights.
Previously predicted as the future of the British fight scene, he now is positioned trying to realize his potential - maybe in what feels like a final roll of the dice - under new promotional backing.
He started carefully, with the two boxers ready to take risks in a calculated initial rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in two years ago after damaging his hand mid-fight, was making just his second appearance at light-heavyweight.
By the halfway stage, Parker had made it competitive, causing problems for Buatsi, who operated with a tight guard to narrowly win the better rounds.
The audience's tolerance decreased amid constant clinching, with Parker slipping to the canvas various instances during close exchanges.
Parker ended stronger, but several stanzas were hard to judge. Although, ex-champion Carl Frampton - working with Dazn - had it strongly leaning for Parker, scoring for Buatsi just only one stanza.
The victor progresses, but with the likes of heavy hitter Benavidez, and Eastern European fighter Bivol leading the category, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Memorial Fuels All-Manchester Fight
In the supporting bout, Arthur defeated Bradley Rea by divided decision in a excellent all-Manchester battle to capture the European light heavyweight title.
The arena became quiet for a deeply poignant homage ahead of the contest as ten bells sounded in memory of Manchester's own Ricky Hatton, who passed away 46 years old in last month.
It was the first major televised fight night in the city since the passing of Hatton. As the last bell ended, the stillness transitioned to a roar of "there's just one Hatton".
The competition within the ring reflected the emotion. Arthur was floored by a left hook directly to the chin in the second round, standing uncertainly to his feet on the referee's count.
But the traditional red shorts of Arthur soon came forward again. Across the ring, Rea - in rival City blue - was marked up soon, significant bruising forming under his face as the fighters traded hard punches.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, matched the intensity with powerful upward punches as the pace hardly diminished over the full distance.
The victor barely secured the result with official scores of draw, narrow lead and 115-112, but both fighters were relentless and brave, exactly what their inspiration Hatton represented.
