British light-heavyweight Buatsi earned a win yet underwhelming start to his career under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a split decision in a tight 10-round fight at the Manchester venue.
But Parker found success, landing some telling individual punches in a fight where the styles didn't complement each other, with excessive holding spoiling the action.
The officials scored it 96-94 in favor to the winner, with the remaining official a 95-95.
"It wasn't my finest display but what defines champions? They succeed," Buatsi commented in the ringside as he was faced boos from the spectators.
The British fighter returned to winning ways after his maiden career setback - an thrilling contest with Callum Smith in last winter - to secure a 20th pro success.
Several boxing experts believed the challenger, thirty-one, merited the victory but the British boxer experienced the second defeat of his 28-bout professional journey.
Buatsi Earns Victory in Dull Affair
The fighter's advancement has plateaued in recent years through promotional disputes and long spells of inactivity.
Previously predicted as the future of UK boxing, he now finds himself trying to realize his potential - maybe in what seems like a ultimate opportunity - under recent support.
He started carefully, with the two boxers prepared to overcommit in a cagey early stages.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after damaging his hand in the contest, was participating in his second appearance at light-heavyweight.
By the middle rounds, Parker had made it competitive, causing problems for the favorite, who utilized a solid defense to barely take the better stanzas.
The audience's tolerance decreased during constant clinching, with Parker slipping to the canvas various instances during grappling.
Parker ended stronger, but multiple periods were difficult to score. Nevertheless, previous titleholder Carl Frampton - commentating on the broadcast network - had it strongly leaning for Parker, giving Buatsi just only one stanza.
Buatsi progresses, but with the caliber of power puncher David Benavidez, and Russian champion Bivol leading the category, the road to world glory - judging by this showing - still seems difficult.
Hatton Tribute Motivates Local Rivalry Contest
In the supporting bout, Lyndon Arthur defeated Rea by close verdict in a excellent local rivalry fight to secure the European light-heavyweight belt.
The stadium grew hushed for a extremely emotional homage before the fight as ten bells sounded in remembrance of the city's native Ricky Hatton, who died 46 years old in September.
It was the premier televised fight night in the urban center since the passing of Hatton. As the concluding sound ended, the silence became a roar of "there's only one Hatton".
The action during the bout equaled the feeling. Arthur was sent to the canvas by a powerful punch squarely on the chin in the second round, getting up unsteadily to his position on the eight count.
But the Manchester United-red shorts of the fighter soon pressed ahead again. On the opposite side, Rea - in rival City blue - was damaged early, noticeable damage developing under his orbital area as the fighters traded punishing blows.
Rea, 27, delivered effective strikes but Arthur, thirty-four, matched the intensity with stinging uppercuts as the action hardly diminished over twelve rounds.
The victor edged the verdict with official scores of draw, 115-113 and wider gap, but the two athletes were determined and courageous, all the qualities their inspiration Hatton stood for.
