UK light-heavyweight Buatsi secured a victory yet disappointing start to life under recent management with a decision win over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a close ten-round bout at the boxing arena.
But the challenger landed significant shots, landing some telling single shots in a contest where the techniques didn't complement each other, with frequent clinching disrupting the fight.
Two judges had it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my best showing but what defines champions? They win," the victor stated in the ringside as he was met by negative reactions from the crowd.
The British fighter got back to winning after his first professional defeat - an epic battle with Callum Smith in February - to earn a 20th career victory.
Various analysts thought Parker, 31, merited the win but the British boxer suffered the second loss of his 28-bout career.
Buatsi Secures Triumph in Uneventful Encounter
His career development has stalled in recent years through contract problems and extended periods of inactivity.
Once tipped as the future of the British fight scene, he now stands trying to fulfil that early promise - perhaps in what appears to be a ultimate opportunity - under new promotional backing.
He opened tentatively, with the two boxers willing to overcommit in a calculated opening few rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in 2022 after damaging his hand in the contest, was participating in his second fight at light-heavyweight.
By the fight's midpoint, the underdog had made it competitive, creating difficulties for the favorite, who worked behind a solid defense to narrowly win the better periods.
The crowd's patience wore thin during constant clinching, with the challenger slipping to the canvas multiple occasions during grappling.
Parker ended stronger, but multiple periods were hard to judge. Nevertheless, ex-champion Carl Frampton - working with Dazn - had it significantly in favor for Parker, giving Buatsi just one round.
The victor advances, but with the likes of heavy hitter Benavidez, and Russian star Bivol ruling the division, the road to world glory - based on this performance - still looks steep.
Ricky Hatton Tribute Fuels Local Rivalry Contest
In the co-main event, Arthur defeated Bradley Rea by close verdict in a outstanding all-Manchester fight to secure the European light-heavyweight title.
The stadium fell silent for a extremely emotional homage before the fight as 10 bells sounded in memory of Manchester's own Hatton, who succumbed aged 46 in recently.
It was the first major televised match card in the city since Hatton's death. As the last bell disappeared, the quiet gave way to a chant of "only one Hatton".
The fighting inside the ropes reflected the feeling. Arthur was sent to the canvas by a left hook flush on the jaw in the second round, standing uncertainly to his feet on the referee's count.
But the traditional red shorts of the fighter soon came forward again. From the other corner, Rea - in rival City blue - was marked up early, noticeable damage appearing under his orbital area as the pair exchanged hard punches.
The challenger, twenty-seven, delivered effective strikes but the champion, 34, responded in kind with effective uppercuts as the intensity seldom decreased over the full distance.
Arthur barely secured the result with judges scorecards of 114-114, narrow lead and wider gap, but the two athletes were relentless and courageous, exactly what their idol Hatton stood for.
