UK light-heavyweight Buatsi earned a win yet underwhelming beginning to life under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a split decision in a competitive ten-round bout at the Manchester venue.
But the challenger had his moments, delivering effective clean blows in a contest where the approaches failed to mesh, with constant grabbing marring the contest.
The officials judged it 96-94 to the winner, with the third a 95-95.
"It wasn't my greatest display but how do champions respond? They succeed," the victor commented in the arena as he was met by negative reactions from the audience.
The British fighter bounced back after his maiden career setback - an memorable fight with his previous opponent in February - to secure a 20th pro success.
Various analysts believed Parker, 31, merited the win but the Derbyshire fighter endured the second loss of his 28-match career.
Buatsi Earns Win in Slow Encounter
His career development has stalled in the past few years through contract problems and considerable time of inactivity.
Previously predicted as the future of UK boxing, he now stands trying to realize his potential - maybe in what appears to be a ultimate opportunity - under recent support.
He started carefully, with both competitors willing to overcommit in a calculated opening few rounds.
Parker, whose sole prior loss came at the lower weight class against his former opponent in 2022 after breaking his hand during the bout, was making just his second appearance at light-heavyweight.
By the fight's midpoint, Parker had kept it close, making life awkward for the favorite, who operated with a solid defense to narrowly win the better rounds.
The spectators' acceptance diminished during constant clinching, with the challenger slipping to the canvas multiple occasions during grappling.
The underdog closed well, but multiple periods were challenging to assess. Although, previous titleholder the boxing analyst - commentating on the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just only one stanza.
The victor progresses, but with the caliber of heavy hitter Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Tribute Inspires Local Rivalry Contest
In the co-main event, Lyndon Arthur beat Rea by divided decision in a excellent Manchester vs Manchester battle to secure the European light-heavyweight belt.
The stadium fell silent for a profoundly moving tribute ahead of the contest as ten bells sounded in remembrance of Manchester's own Ricky Hatton, who succumbed aged 46 in September.
It was the premier televised match card in the urban center since Hatton's death. As the last bell faded, the quiet became a roar of "there's only one Ricky Hatton".
The competition during the bout reflected the emotion. The eventual winner was knocked down by a left hook flush on the chin in the initial stage, getting up unsteadily to his position on the eight count.
But the Manchester United-red attire of Arthur soon came forward again. From the other corner, Rea - in rival City blue - was damaged early, significant bruising appearing under his face as the fighters traded powerful shots.
The challenger, 27, delivered effective strikes but the champion, 34, matched the intensity with effective uppercuts as the intensity rarely dropped over twelve rounds.
The victor narrowly won the verdict with official scores of 114-114, 115-113 and 115-112, but both fighters were relentless and full of heart, all the qualities their hero Hatton stood for.
