UK light-heavyweight Buatsi secured a victory yet underwhelming start to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a majority decision in a tight 10-round bout at the Co-op Live Arena.
But the challenger found success, delivering effective clean blows in a contest where the approaches didn't complement each other, with excessive holding spoiling the fight.
Two judges had it 96-94 for to the winner, with the final judge a even score.
"It wasn't my finest display but how do champions respond? They prevail," Buatsi said in the ringside as he was met by boos from the crowd.
The British fighter bounced back after his first professional defeat - an thrilling contest with his previous opponent in last winter - to secure a twentieth professional win.
Several boxing experts believed Parker, thirty-one, merited the decision but the contender suffered the second setback of his 28-fight career.
Buatsi Earns Win in Dull Encounter
The fighter's advancement has plateaued in recent times through contract problems and extended periods of inactivity.
Previously predicted as the next star of British boxing, he now is positioned trying to achieve expectations - perhaps in what feels like a ultimate opportunity - under recent support.
He started carefully, with neither fighter willing to overcommit in a cagey opening few rounds.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in 2022 after breaking his hand mid-fight, was competing in only his second appearance at light heavyweight.
By the fight's midpoint, the underdog had stayed in contention, causing problems for Buatsi, who utilized a tight guard to edge the more effective periods.
The audience's tolerance wore thin amid constant clinching, with Parker going down several times during clinches.
The underdog ended stronger, but many rounds were difficult to score. Nevertheless, ex-champion Carl Frampton - providing analysis for the broadcast network - had it particularly wide for the challenger, awarding Buatsi just only one stanza.
Buatsi moves on, but with the presence of power puncher David Benavidez, and Eastern European fighter Bivol dominating the weight class, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Memorial Motivates Local Rivalry Battle
In the secondary fight, Lyndon Arthur beat Rea by split-decision in a terrific Manchester vs Manchester fight to secure the continental championship.
The arena fell silent for a deeply poignant memorial before the fight as ten bells sounded in remembrance of local hero Ricky Hatton, who passed away aged 46 in recently.
It was the premier televised boxing event in the location since Hatton's passing. As the last bell faded, the quiet transitioned to a roar of "there's just one Ricky Hatton".
The action during the bout reflected the sentiment. The eventual winner 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 red-colored shorts of the fighter soon came forward again. On the opposite side, the opponent - in opposing blue - was marked up early, noticeable damage forming under his eye as the duo exchanged punishing blows.
Rea, 27, landed power punches but the champion, 34, fought fire with fire with powerful upward punches as the pace rarely dropped over 12 rounds.
The victor edged the decision with scoring tallies of draw, narrow lead and wider gap, but each competitor were persistent and courageous, everything their hero the legend embodied.
