UK light heavyweight Buatsi made a winning yet unimpressive beginning to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a split decision in a tight 10-round fight at the Manchester venue.
But the challenger landed significant shots, delivering effective individual punches in a match where the styles never quite clicked, with frequent clinching disrupting the fight.
The scoring judges scored it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my finest performance but what do champions do? They succeed," the victor said in the ring as he was faced boos from the audience.
The British fighter got back to winning after his initial pro loss - an thrilling contest with his previous opponent in last winter - to secure a 20th pro success.
Various analysts believed Parker, 31, deserved the victory but the Derbyshire fighter endured the second setback of his 28-match career.
The Champion Claims Win in Uneventful Encounter
The fighter's advancement has stalled in the past few years through management issues and long spells of lack of fights.
Once tipped as the next star of British boxing, he now stands trying to fulfil that early promise - maybe in what feels like a ultimate opportunity - under fresh management.
He opened tentatively, with the two boxers ready to take risks in a calculated initial rounds.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in two years ago after breaking his hand during the bout, was participating in his second fight at light heavyweight.
By the halfway stage, the underdog had made it competitive, making life awkward for Buatsi, who worked behind a tight guard to barely take the better rounds.
The audience's tolerance wore thin amid regular grabbing, with Parker going down multiple occasions during grappling.
Parker closed well, but multiple periods were challenging to assess. Nevertheless, ex-champion Carl Frampton - commentating on Dazn - had it significantly in favor for the challenger, giving Buatsi just only one stanza.
Buatsi progresses, but with the presence of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the path to championship success - on this evidence - still appears challenging.
Ricky Hatton Tribute Motivates Manchester Derby Contest
In the supporting bout, Arthur beat Bradley Rea by split-decision in a outstanding all-Manchester battle to secure the continental championship.
The arena fell silent for a deeply poignant homage ahead of the contest as 10 bells rang in honor of Manchester's own Ricky Hatton, who succumbed aged 46 in September.
It was the premier televised boxing event in the location since Hatton's death. As the last bell faded, the quiet became a shout of "there's only one Hatton".
The action inside the ropes matched the emotion. Arthur was floored by a significant blow flush on the face in the initial stage, rising shakily to his feet on the referee's count.
But the red-colored attire of Arthur soon advanced again. On the opposite side, the opponent - in competitive blue - was marked up early, heavy swelling developing under his orbital area as the pair exchanged punishing blows.
The challenger, 27, delivered effective strikes but the champion, 34, responded in kind with effective uppercuts as the intensity seldom decreased over the full distance.
Arthur narrowly won the decision with scoring tallies of 114-114, 115-113 and 115-112, but each competitor were relentless and courageous, exactly what their hero the legend stood for.
