British light heavyweight Buatsi secured a victory yet underwhelming debut to life under recent management with a decision win over Parker in the Manchester arena.
The Olympic medal winner's punch output seemingly secured him a split decision in a competitive 10-round bout at the Manchester venue.
But Parker had his moments, connecting with powerful single shots in a match where the styles never quite clicked, with excessive holding disrupting the action.
Two judges scored it 96-94 for to the winner, with the third a 95-95.
"It wasn't my best display but how do champions respond? They prevail," the victor stated in the ringside as he was met by boos from the spectators.
The British fighter bounced back after his maiden career setback - an epic battle with Callum Smith in February - to secure a 20th pro success.
Various analysts believed Parker, 31, merited the win but the Derbyshire fighter suffered the second loss of his 28-match career.
The Champion Earns Victory in Uneventful Encounter
Buatsi's progress has slowed in recent years through management issues and extended periods of lack of fights.
Earlier forecast as the future of British boxing, he now is positioned trying to fulfil that early promise - perhaps in what seems like a ultimate opportunity - under new promotional backing.
He began cautiously, with neither fighter prepared to be aggressive in a calculated initial rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in two years ago after suffering a hand injury mid-fight, was competing in only his second bout at light-heavyweight.
By the middle rounds, Parker had stayed in contention, making life awkward for Buatsi, who operated with a solid defense to barely take the cleaner rounds.
The audience's tolerance decreased because of frequent holding, with Parker falling to the floor various instances during close exchanges.
Parker ended stronger, but multiple periods were challenging to assess. Although, former world champion Carl Frampton - working with Dazn - had it significantly in favor for Parker, scoring for Buatsi just a single round.
Buatsi progresses, but with the presence of heavy hitter Benavidez, and Russian star Dmitry Bivol leading the category, the journey to titles - on this evidence - still appears challenging.
Hatton Memorial Fuels Manchester Derby Contest
In the co-main event, Arthur beat Rea by close verdict in a terrific all-Manchester fight to win the European light-heavyweight belt.
The venue grew hushed for a deeply poignant tribute prior to the bout as ten bells tolled in honor of local hero Hatton, who passed away at age 46 in recently.
It was the first major televised boxing event in the location since Hatton's passing. As the final chime disappeared, the stillness became a roar of "there's just one Hatton".
The action during the bout equaled the emotion. The eventual winner was knocked down by a powerful punch directly to the chin in the second round, getting up unsteadily to his position on the referee's count.
But the red-colored shorts of the fighter soon pressed ahead again. From the other corner, Rea - in opposing blue - was injured quickly, heavy swelling developing under his face as the pair landed powerful shots.
The challenger, 27, delivered effective strikes but the champion, thirty-four, fought fire with fire with effective uppercuts as the pace rarely dropped over 12 rounds.
Arthur edged the verdict with official scores of 114-114, close margin and 115-112, but the two athletes were relentless and brave, all the qualities their idol Hatton represented.
