UK light heavyweight Buatsi secured a victory yet underwhelming start to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level seemingly secured him a judges' verdict in a tight ten-round bout at the boxing arena.
But Parker had his moments, landing some telling single shots in a match where the techniques never quite clicked, with excessive holding spoiling the fight.
The officials judged it 96-94 for to the winner, with the final judge a 95-95 draw.
"It wasn't my finest display but what defines champions? They win," the victor stated in the ringside as he was faced disapproval from the audience.
The Londoner returned to winning ways after his maiden career setback - an epic battle with his previous opponent in February - to claim a 20th pro success.
Various analysts believed Parker, thirty-one, should have been awarded the decision but the contender endured the second loss of his 28-fight career.
Buatsi Earns Victory in Slow Affair
The fighter's advancement has plateaued in the past few years through promotional disputes and long spells of inactivity.
Once tipped as the next star of British boxing, he now is positioned trying to realize his potential - possibly in what appears to be a ultimate opportunity - under fresh management.
He opened tentatively, with neither fighter ready to be aggressive in a tentative initial rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after damaging his hand during the bout, was making just his second fight at the higher weight class.
By the fight's midpoint, the underdog had stayed in contention, causing problems for the favorite, who operated with a protective stance to edge the more effective periods.
The spectators' acceptance wore thin because of constant clinching, with the challenger falling to the floor several times during clinches.
Parker closed well, but multiple periods were challenging to assess. However, former world champion Carl Frampton - working with the broadcast network - had it strongly leaning for the challenger, giving Buatsi just one round.
Buatsi advances, but with the likes of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the path to championship success - judging by this showing - still appears challenging.
Ricky Hatton Tribute Fuels Local Rivalry Battle
In the secondary fight, Lyndon Arthur beat Bradley Rea by divided decision in a excellent local rivalry contest to win the European light heavyweight championship.
The arena grew hushed for a extremely emotional memorial before the fight as a bell ceremony rang in honor of Manchester's own Hatton, who succumbed aged 46 in September.
It was the premier televised boxing event in the location since the passing of Hatton. As the concluding sound ended, the silence gave way to a shout of "there's only one Hatton".
The fighting during the bout matched the feeling. Arthur was sent to the canvas by a powerful punch directly to the face in the second round, rising shakily to his position on the count of eight.
But the Manchester United-red equipment of the fighter soon advanced again. On the opposite side, the opponent - in competitive blue - was damaged quickly, significant bruising forming under his orbital area as the pair exchanged punishing blows.
The challenger, 27, connected with strong blows but Arthur, thirty-four, responded in kind with powerful upward punches as the action seldom decreased over twelve rounds.
The victor edged the result with official scores of draw, close margin and clearer margin, but both fighters were persistent and full of heart, all the qualities their inspiration the legend embodied.
