UK light-heavyweight Buatsi made a winning yet underwhelming beginning to his career under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a majority decision in a close 10-round bout at the Manchester venue.
But the challenger had his moments, landing some telling individual punches in a match where the techniques never quite clicked, with constant grabbing disrupting the contest.
Two judges judged it 96-94 in favor to the winner, with the remaining official a 95-95.
"It wasn't my best performance but how do champions respond? They prevail," Buatsi stated in the ringside as he was met by disapproval from the crowd.
The Londoner returned to winning ways after his first professional defeat - an memorable fight with his previous opponent in last winter - to secure a twentieth professional win.
Several boxing experts thought the challenger, 31, merited the win but the contender experienced the second defeat of his 28-match career.
Buatsi Earns Triumph in Dull Match
The fighter's advancement has slowed in recent times through management issues and long spells of inactivity.
Earlier forecast as the next star of UK boxing, he now finds himself trying to fulfil that early promise - possibly in what feels like a final roll of the dice - under recent support.
He began cautiously, with both competitors ready to be aggressive in a cagey opening few rounds.
The challenger, whose single earlier setback 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 fight's midpoint, the underdog had stayed in contention, creating difficulties for the favorite, who utilized a tight guard to barely take the cleaner rounds.
The audience's tolerance diminished during regular grabbing, with Parker falling to the floor various instances during grappling.
The underdog finished the stronger, but many rounds were challenging to assess. Although, former world champion Carl Frampton - commentating on Dazn - had it strongly leaning for the challenger, giving Buatsi just a single round.
The victor moves on, but with the presence of power puncher Benavidez, and Russian star Dmitry Bivol leading the category, the road to world glory - on this evidence - still looks steep.
Ricky Hatton Homage Inspires Manchester Derby Contest
In the co-main event, Lyndon Arthur overcame Rea by split-decision in a terrific all-Manchester contest to capture the European light-heavyweight title.
The venue fell silent for a deeply poignant homage ahead of the contest as a bell ceremony sounded in honor of the city's native Hatton, who succumbed 46 years old in September.
It was the initial significant broadcast boxing event in the city since Hatton's death. As the last bell ended, the stillness gave way to a shout of "only one Hatton".
The competition inside the ropes reflected the emotion. Arthur was knocked down by a significant blow flush on the chin in the initial stage, rising shakily to his footing on the referee's count.
But the Manchester United-red shorts of Arthur soon pressed ahead again. On the opposite side, the opponent - in rival City blue - was injured early, significant bruising appearing under his eye as the fighters exchanged punishing blows.
The challenger, twenty-seven, connected with strong blows but the champion, 34, matched the intensity with stinging uppercuts as the pace rarely dropped over 12 rounds.
The victor barely secured the decision with scoring tallies of draw, narrow lead and clearer margin, but the two athletes were persistent and brave, all the qualities their hero Hatton represented.
