UK light-heavyweight Buatsi secured a victory yet unimpressive beginning to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a majority decision in a close 10-round fight at the boxing arena.
But Parker found success, delivering effective clean blows in a fight where the techniques didn't complement each other, with excessive holding disrupting the action.
The scoring judges scored it 96-94 to Buatsi, with the final judge a even score.
"It wasn't my greatest showing but what do champions do? They succeed," Buatsi said in the arena as he was faced negative reactions from the spectators.
The Londoner got back to winning after his first professional defeat - an memorable fight with his previous opponent in February - to earn a 20th career victory.
Several boxing experts felt the challenger, 31, deserved the decision but the British boxer endured the second loss of his 28-fight career.
Buatsi Claims Victory in Dull Encounter
The fighter's advancement has plateaued in the past few years through promotional disputes and considerable time of inactivity.
Earlier forecast as the rising talent of British boxing, he now finds himself trying to realize his potential - maybe in what appears to be a last chance - under recent support.
He started carefully, with both competitors willing to be aggressive in a tentative early stages.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in 2022 after damaging his hand during the bout, was competing in only his second bout at light heavyweight.
By the halfway stage, the underdog had kept it close, creating difficulties for the favorite, who operated with a solid defense to edge the more effective rounds.
The spectators' acceptance wore thin during constant clinching, with the challenger falling to the floor several times during grappling.
The underdog finished the stronger, but many rounds were hard to judge. However, previous titleholder the boxing analyst - providing analysis for Dazn - had it significantly in favor for Parker, scoring for Buatsi just a single round.
The victor advances, but with the caliber of heavy hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the path to championship success - based on this performance - still looks steep.
Ricky Hatton Homage Inspires Local Rivalry Contest
In the secondary fight, Lyndon Arthur defeated Rea by close verdict in a terrific all-Manchester fight to secure the European light-heavyweight title.
The venue grew hushed for a profoundly moving homage before the fight as a bell ceremony rang in honor of Manchester's own Ricky Hatton, who passed away aged 46 in last month.
It was the premier televised boxing event in the urban center since Hatton's death. As the concluding sound ended, the silence transitioned to a roar of "only one Ricky Hatton".
The fighting during the bout reflected the feeling. Arthur was floored by a significant blow squarely on the chin in the initial stage, getting up unsteadily to his feet on the eight count.
But the red-colored shorts of Arthur soon advanced again. Across the ring, Rea - in rival City blue - was injured quickly, heavy swelling developing under his orbital area as the pair exchanged powerful shots.
Rea, 27, connected with strong blows but Arthur, 34, matched the intensity with stinging uppercuts as the intensity rarely dropped over 12 rounds.
Arthur barely secured the verdict with official scores of even score, close margin and clearer margin, but the two athletes were persistent and full of heart, exactly what their idol the legend stood for.
