UK light-heavyweight Buatsi secured a victory yet underwhelming beginning to his career under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic medal winner's punch output appeared to earn him a majority decision in a close 10-round fight at the Co-op Live Arena.
But the challenger landed significant shots, connecting with powerful individual punches in a contest where the techniques didn't complement each other, with frequent clinching spoiling the fight.
The officials judged it 96-94 in favor to the winner, with the third a 95-95 draw.
"It wasn't my finest display but what do champions do? They prevail," the victor said in the ringside as he was greeted with negative reactions from the audience.
The British fighter bounced back after his first professional defeat - an epic battle with his previous opponent in last winter - to secure a twentieth professional win.
Several boxing experts believed Parker, 31, should have been awarded the win but the British boxer experienced the second defeat of his 28-match career.
Buatsi Claims Win in Dull Affair
The fighter's advancement has plateaued in recent times through management issues and considerable time of inactivity.
Earlier forecast as the future of the British fight scene, he now is positioned trying to achieve expectations - perhaps in what appears to be a final roll of the dice - under recent support.
He opened tentatively, with both competitors ready to take risks in a calculated early stages.
Parker, whose sole prior loss came at super middleweight against John Ryder in 2022 after damaging his hand mid-fight, was competing in only his second fight at the higher weight class.
By the middle rounds, Parker had kept it close, creating difficulties for the favorite, who operated with a tight guard to barely take the more effective stanzas.
The spectators' acceptance diminished because of frequent holding, with Parker going down various instances during close exchanges.
Parker ended stronger, but several stanzas were difficult to score. Nevertheless, previous titleholder the boxing analyst - working with the broadcast network - had it particularly wide for the challenger, giving Buatsi just only one stanza.
Buatsi advances, but with the caliber of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the journey to titles - based on this performance - still seems difficult.
Hatton Memorial Fuels Manchester Derby Battle
In the secondary fight, Arthur overcame Rea by divided decision in a outstanding Manchester vs Manchester contest to secure the European light heavyweight belt.
The arena became quiet for a deeply poignant homage before the fight as a bell ceremony tolled in remembrance of Manchester's own Ricky Hatton, who succumbed 46 years old in last month.
It was the initial significant broadcast fight night in the location since the passing of Hatton. As the last bell disappeared, the stillness gave way to a shout of "there's just one Hatton".
The competition during the bout matched the sentiment. Arthur was sent to the canvas by a significant blow flush on the chin in the initial stage, rising shakily to his feet on the referee's count.
But the Manchester United-red equipment of the fighter soon pressed ahead again. On the opposite side, Rea - in rival City blue - was marked up quickly, heavy swelling appearing under his face as the duo exchanged hard punches.
Rea, 27, delivered effective strikes but Arthur, 34, responded in kind with stinging uppercuts as the intensity rarely dropped over twelve rounds.
The victor barely secured the verdict with judges scorecards of draw, close margin and 115-112, but the two athletes were determined and brave, exactly what their hero Hatton embodied.
