UK light-heavyweight Buatsi earned a win yet unimpressive beginning to his career under recent management with a decision win over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a split decision in a tight 10-round fight at the boxing arena.
But the challenger found success, connecting with powerful single shots in a match where the approaches never quite clicked, with excessive holding spoiling the fight.
The scoring judges judged it 96-94 in favor to Buatsi, with the third a 95-95.
"It wasn't my greatest showing but what do champions do? They succeed," Buatsi said in the arena as he was greeted with boos from the spectators.
The British fighter got back to winning after his first professional defeat - an epic battle with his previous opponent in last winter - to secure a 20th career victory.
Some ringside observers believed Parker, 31, should have been awarded the decision but the contender suffered the second setback of his 28-bout career.
The Champion Claims Triumph in Dull Encounter
Buatsi's progress has stalled in the past few years through contract problems and extended periods of lack of fights.
Earlier forecast as the rising talent of British boxing, he now is positioned trying to realize his potential - perhaps in what feels like a last chance - under recent support.
He opened tentatively, with neither fighter prepared to be aggressive in a cagey opening few rounds.
The challenger, whose sole prior loss came at super middleweight against his former opponent in 2022 after damaging his hand in the contest, was competing in only his second fight at light-heavyweight.
By the halfway stage, Parker had kept it close, causing problems for the favorite, who worked behind a protective stance to narrowly win the cleaner periods.
The spectators' acceptance wore thin because of frequent holding, with Parker falling to the floor multiple occasions during close exchanges.
Parker closed well, but several stanzas were challenging to assess. Although, former world champion the boxing analyst - commentating on the broadcast network - had it particularly wide for the challenger, awarding Buatsi just a single round.
The victor advances, but with the presence of hard hitter Benavidez, and Eastern European fighter Bivol dominating the weight class, the journey to titles - on this evidence - still looks steep.
Hatton Tribute Fuels All-Manchester Fight
In the supporting bout, Lyndon Arthur overcame Rea by split-decision in a excellent all-Manchester contest to win the European light heavyweight belt.
The stadium became quiet for a deeply poignant memorial ahead of the contest as ten bells rang in remembrance of the city's native Ricky Hatton, who passed away 46 years old in last month.
It was the initial significant broadcast fight night in the city since Hatton's passing. As the last bell disappeared, the silence became a shout of "there's only one Hatton".
The competition during the bout matched the emotion. The eventual winner was floored by a significant blow directly to the jaw 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. Across the ring, the opponent - in rival City blue - was injured soon, significant bruising developing under his face as the duo exchanged punishing blows.
Rea, twenty-seven, landed power punches but the champion, 34, matched the intensity with powerful upward punches as the pace seldom decreased over the full distance.
The victor edged the verdict with scoring tallies of even score, close margin and 115-112, but both fighters were determined and courageous, everything their inspiration the legend stood for.
