British light-heavyweight Buatsi earned a win yet unimpressive debut to his career under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a majority decision in a close ten-round contest at the Manchester venue.
But Parker had his moments, delivering effective clean blows in a contest where the techniques failed to mesh, with frequent clinching marring the action.
Two judges scored it 96-94 for to Buatsi, with the final judge a 95-95 draw.
"It wasn't my finest performance but what do champions do? They prevail," Buatsi commented in the ring as he was greeted with boos from the audience.
The Londoner returned to winning ways after his initial pro loss - an thrilling contest with Callum Smith in February - to earn a 20th pro success.
Some ringside observers believed Parker, thirty-one, should have been awarded the victory but the contender endured the second defeat of his 28-match career.
The Champion Earns Triumph in Uneventful Encounter
His career development has stalled in recent times through contract problems and long spells of lack of fights.
Earlier forecast as the rising talent of British boxing, he now finds himself trying to fulfil that early promise - possibly in what appears to be a ultimate opportunity - under recent support.
He opened tentatively, with neither fighter prepared to overcommit in a tentative early stages.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in 2022 after breaking his hand in the contest, was participating in his second fight at light-heavyweight.
By the fight's midpoint, the underdog had stayed in contention, making life awkward for Buatsi, who utilized a protective stance to narrowly win the better periods.
The audience's tolerance wore thin during constant clinching, with Parker slipping to the canvas multiple occasions during grappling.
Parker ended stronger, but many rounds were challenging to assess. However, previous titleholder Carl Frampton - providing analysis for Dazn - had it strongly leaning for Parker, scoring for Buatsi just only one stanza.
The victor progresses, but with the caliber of hard hitter David Benavidez, and Russian champion Bivol dominating the weight class, the path to championship success - judging by this showing - still looks steep.
Ricky Hatton Homage Fuels Local Rivalry Battle
In the co-main event, Arthur overcame Rea by divided decision in a terrific Manchester vs Manchester battle to capture the continental championship.
The stadium grew hushed for a deeply poignant tribute prior to the bout as 10 bells rang in honor of Manchester's own Hatton, who died at age 46 in last month.
It was the initial significant broadcast boxing event in the location since Hatton's death. As the concluding sound faded, the stillness became a chant of "there's only one Hatton".
The action during the bout equaled the feeling. Arthur was knocked down by a powerful punch flush on the chin in the initial stage, standing uncertainly to his feet on the referee's count.
But the red-colored equipment of Arthur soon came forward again. Across the ring, the opponent - in opposing blue - was marked up early, significant bruising appearing under his orbital area as the duo landed punishing blows.
The challenger, 27, landed power punches but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the intensity hardly diminished over 12 rounds.
Arthur edged the result with judges scorecards of 114-114, narrow lead and wider gap, but each competitor were relentless and full of heart, exactly what their idol the legend stood for.
