British light heavyweight Buatsi secured a victory yet unimpressive debut to his career under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's punch output likely gained him a majority decision in a competitive 10-round contest at the Co-op Live Arena.
But the challenger had his moments, delivering effective single shots in a match where the techniques didn't complement each other, with excessive holding spoiling the fight.
The officials judged it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my greatest display but what do champions do? They prevail," the victor said in the ring as he was faced disapproval from the crowd.
The Londoner bounced back after his first professional defeat - an epic battle with his previous opponent in February - to secure a twentieth professional win.
Some ringside observers believed Parker, 31, deserved the decision but the contender suffered the second defeat of his 28-match professional journey.
The Champion Earns Triumph in Slow Encounter
The fighter's advancement has plateaued in recent years through management issues and extended periods of inactivity.
Previously predicted as the next star of UK boxing, he now stands trying to achieve expectations - perhaps in what appears to be a last chance - under recent support.
He started carefully, with neither fighter ready to overcommit in a tentative early stages.
Parker, whose single earlier setback came at the lower weight class against John Ryder in 2022 after breaking his hand in the contest, was participating in his second appearance at light-heavyweight.
By the halfway stage, the underdog had stayed in contention, causing problems for the favorite, who worked behind a protective stance to edge the cleaner rounds.
The crowd's patience diminished amid regular grabbing, with the challenger slipping to the canvas multiple occasions during close exchanges.
Parker finished the stronger, but many rounds were hard to judge. Nevertheless, previous titleholder Carl Frampton - providing analysis for Dazn - had it significantly in favor for the challenger, giving Buatsi just only one stanza.
Buatsi progresses, but with the likes of power puncher Benavidez, and Russian champion Bivol leading the category, the road to world glory - judging by this showing - still appears challenging.
Ricky Hatton Memorial Inspires All-Manchester Fight
In the supporting bout, Lyndon Arthur beat Rea by divided decision in a outstanding all-Manchester battle to secure the continental championship.
The venue fell silent for a extremely emotional memorial before the fight as ten bells sounded in memory of Manchester's own Hatton, who succumbed 46 years old in September.
It was the first major televised fight night in the urban center since the passing of Hatton. As the last bell ended, the stillness gave way to a roar of "only one Ricky Hatton".
The action inside the ropes matched the feeling. Arthur was knocked down by a powerful punch flush on the face in the initial stage, getting up unsteadily to his footing on the eight count.
But the red-colored attire of Arthur soon advanced again. Across the ring, Rea - in rival City blue - was injured soon, significant bruising developing under his orbital area as the duo landed punishing blows.
The challenger, twenty-seven, connected with strong blows but Arthur, thirty-four, responded in kind with effective uppercuts as the intensity seldom decreased over twelve rounds.
Arthur edged the decision with judges scorecards of draw, close margin and 115-112, but the two athletes were determined and brave, exactly what their idol the legend stood for.
