UK light heavyweight Buatsi earned a win yet disappointing beginning to life under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a judges' verdict in a close 10-round bout at the boxing arena.
But the challenger found success, connecting with powerful clean blows in a fight where the approaches failed to mesh, with constant grabbing disrupting the contest.
The officials judged it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my finest display but what defines champions? They prevail," the victor stated in the ring as he was faced disapproval from the audience.
The Londoner bounced back after his maiden career setback - an memorable fight with Callum Smith in February - to earn a 20th pro success.
Several boxing experts believed Parker, thirty-one, merited the decision but the Derbyshire fighter experienced the second setback of his 28-match professional journey.
The Champion Secures Victory in Slow Match
Buatsi's progress has plateaued in the past few years through promotional disputes and considerable time of inactivity.
Earlier forecast as the future of British boxing, he now stands trying to fulfil that early promise - maybe in what feels like a ultimate opportunity - under recent support.
He began cautiously, with neither fighter ready to overcommit in a cagey opening few rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after breaking his hand mid-fight, was participating in his second appearance at light-heavyweight.
By the middle rounds, Parker had stayed in contention, creating difficulties for the favorite, who utilized a solid defense to barely take the better periods.
The crowd's patience diminished during frequent holding, with the challenger slipping to the canvas multiple occasions during grappling.
The underdog finished the stronger, but several stanzas were challenging to assess. Although, previous titleholder the boxing analyst - working with Dazn - had it particularly wide for Parker, scoring for Buatsi just only one stanza.
Buatsi advances, but with the caliber of hard hitter Benavidez, and Russian star Dmitry Bivol ruling the division, the path to championship success - judging by this showing - still looks steep.
Hatton Memorial Inspires Manchester Derby Contest
In the secondary fight, Lyndon Arthur defeated Bradley Rea by split-decision in a terrific Manchester vs Manchester fight to secure the continental title.
The stadium grew hushed for a extremely emotional memorial ahead of the contest as ten bells sounded in remembrance of the city's native Ricky Hatton, who succumbed aged 46 in recently.
It was the first major televised match card in the location since Hatton's death. As the concluding sound faded, the silence became a chant of "only one Ricky Hatton".
The action inside the ropes equaled the feeling. Arthur was knocked down by a left hook squarely on the jaw in the second round, standing uncertainly to his position on the count of eight.
But the Manchester United-red shorts of the fighter soon pressed ahead again. On the opposite side, the opponent - in opposing blue - was damaged soon, noticeable damage forming under his orbital area as the pair landed powerful shots.
Rea, twenty-seven, connected with strong blows but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the intensity seldom decreased over 12 rounds.
Arthur narrowly won the decision with judges scorecards of even score, 115-113 and wider gap, but the two athletes were relentless and full of heart, exactly what their inspiration the legend stood for.
