UK light-heavyweight Buatsi earned a win yet disappointing beginning to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a majority decision in a tight 10-round fight at the boxing arena.
But the challenger found success, connecting with powerful clean blows in a match where the techniques didn't complement each other, with constant grabbing spoiling the fight.
Two judges judged it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my finest showing but what do champions do? They prevail," the victor said in the arena as he was faced disapproval from the audience.
The Londoner got back to winning after his first professional defeat - an thrilling contest with Callum Smith in February - to claim a twentieth professional win.
Various analysts felt the challenger, thirty-one, deserved the victory but the British boxer endured the second defeat of his 28-fight professional journey.
Buatsi Earns Triumph in Uneventful Match
His career development has stalled in recent years through promotional disputes and considerable time of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now is positioned trying to achieve expectations - possibly in what appears to be a final roll of the dice - under recent support.
He began cautiously, with the two boxers prepared to be aggressive in a tentative early stages.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in 2022 after suffering a hand injury during the bout, was competing in only his second fight at the higher weight class.
By the middle rounds, the underdog had kept it close, causing problems for the favorite, who worked behind a solid defense to barely take the better rounds.
The spectators' acceptance wore thin because of constant clinching, with the challenger falling to the floor various instances during close exchanges.
Parker closed well, but several stanzas were challenging to assess. Although, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it significantly in favor for Parker, giving Buatsi just a single round.
Buatsi progresses, but with the likes of heavy hitter Benavidez, and Russian star Bivol leading the category, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Memorial Fuels Manchester Derby Fight
In the co-main event, Lyndon Arthur overcame Bradley Rea by close verdict in a outstanding all-Manchester fight to capture the European light-heavyweight title.
The arena became quiet for a extremely emotional homage before the fight as ten bells sounded in remembrance of Manchester's own Ricky Hatton, who died 46 years old in September.
It was the first major televised match card in the location since Hatton's death. As the last bell ended, the silence gave way to a chant of "there's just one Ricky Hatton".
The fighting during the bout reflected the feeling. Arthur was knocked down by a significant blow directly to the jaw in the early round, getting up unsteadily to his feet on the count of eight.
But the red-colored attire of the fighter soon came forward again. From the other corner, the opponent - in rival City blue - was marked up early, significant bruising developing under his eye as the duo traded powerful shots.
The challenger, 27, landed power punches but Arthur, 34, fought fire with fire with effective uppercuts as the pace rarely dropped over 12 rounds.
The victor barely secured the decision with judges scorecards of draw, close margin and clearer margin, but the two athletes were determined and courageous, all the qualities their inspiration the legend embodied.
