UK light heavyweight Buatsi made a winning yet disappointing start to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a majority decision in a close 10-round bout at the Manchester venue.
But the challenger found success, landing some telling single shots in a match where the approaches never quite clicked, with constant grabbing disrupting the action.
The scoring judges had it 96-94 in favor to Buatsi, with the third a 95-95.
"It wasn't my best display but what do champions do? They win," the victor said in the ringside as he was greeted with disapproval from the crowd.
The Londoner returned to winning ways after his initial pro loss - an epic battle with his previous opponent in February - to claim a 20th career victory.
Several boxing experts believed Parker, thirty-one, merited the victory but the British boxer suffered the second setback of his 28-bout professional journey.
The Champion Claims Win in Uneventful Affair
His career development has stalled in recent times through management issues and long spells of inactivity.
Once tipped as the rising talent of UK boxing, he now finds himself trying to achieve expectations - perhaps in what appears to be a last chance - under recent support.
He opened tentatively, with the two boxers willing to be aggressive in a cagey initial rounds.
The challenger, whose single earlier setback came at super middleweight against his former opponent in 2022 after damaging his hand mid-fight, was making just his second fight at the higher weight class.
By the halfway stage, the underdog had made it competitive, causing problems for the favorite, who worked behind a solid defense to narrowly win the more effective periods.
The audience's tolerance diminished amid regular grabbing, with the challenger going down multiple occasions during clinches.
Parker ended stronger, but many rounds were challenging to assess. However, ex-champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for the challenger, giving Buatsi just one round.
The victor moves on, but with the presence of hard hitter Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still seems difficult.
Ricky Hatton Memorial Inspires Manchester Derby Battle
In the supporting bout, Arthur beat Rea by split-decision in a excellent Manchester vs Manchester fight to win the continental title.
The stadium fell silent for a deeply poignant memorial before the fight as a bell ceremony tolled in honor of the city's native Hatton, who died aged 46 in September.
It was the first major televised fight night in the urban center since Hatton's death. As the final chime disappeared, the quiet gave way to a roar of "there's just one Ricky Hatton".
The competition inside the ropes equaled the feeling. Arthur was sent to the canvas by a left hook directly to the chin in the early round, standing uncertainly to his feet on the count of eight.
But the red-colored equipment of the fighter soon advanced again. From the other corner, the opponent - in opposing blue - was marked up early, noticeable damage developing under his orbital area as the fighters exchanged punishing blows.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, matched the intensity with effective uppercuts as the action seldom decreased over the full distance.
Arthur narrowly won the decision with judges scorecards of even score, narrow lead and wider gap, but each competitor were persistent and courageous, all the qualities their hero the legend stood for.
