British light heavyweight Buatsi secured a victory yet unimpressive debut to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a close 10-round fight at the Manchester venue.
But Parker landed significant shots, delivering effective individual punches in a contest where the techniques failed to mesh, with excessive holding spoiling the action.
Two judges scored it 96-94 in favor to the winner, with the third a 95-95.
"It wasn't my best performance but what do champions do? They succeed," the victor stated in the arena as he was met by disapproval from the spectators.
The British fighter bounced back after his first professional defeat - an epic battle with his previous opponent in February - to secure a twentieth professional win.
Various analysts believed the challenger, 31, should have been awarded the decision but the British boxer experienced the second loss of his 28-match professional journey.
The Champion Secures Triumph in Uneventful Encounter
The fighter's advancement has slowed in recent times through contract problems and extended periods of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now finds himself trying to achieve expectations - possibly in what appears to be a final roll of the dice - under new promotional backing.
He began cautiously, with neither fighter prepared to be aggressive in a calculated initial rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in two years ago after suffering a hand injury in the contest, was making just his second bout at light heavyweight.
By the halfway stage, Parker had kept it close, creating difficulties for Buatsi, who worked behind a tight guard to barely take the more effective periods.
The spectators' acceptance wore thin during regular grabbing, with the challenger going down various instances during grappling.
The underdog closed well, but multiple periods were difficult to score. Although, ex-champion Carl Frampton - providing analysis for Dazn - had it particularly wide for Parker, scoring for Buatsi just only one stanza.
The victor advances, but with the likes of hard hitter David Benavidez, and Russian star Bivol ruling the division, the road to world glory - judging by this showing - still appears challenging.
Ricky Hatton Tribute Motivates Local Rivalry Fight
In the secondary fight, Arthur beat Bradley Rea by divided decision in a excellent local rivalry fight to capture the continental belt.
The stadium grew hushed for a deeply poignant homage ahead of the contest as a bell ceremony sounded in remembrance of Manchester's own Hatton, who died aged 46 in last month.
It was the initial significant broadcast boxing event in the urban center since the passing of Hatton. As the final chime ended, the silence gave way to a chant of "only one Ricky Hatton".
The fighting within the ring matched the emotion. The eventual winner was knocked down by a left hook directly to the jaw in the second round, standing uncertainly to his footing on the eight count.
But the traditional red shorts of the fighter soon advanced again. From the other corner, Rea - in opposing blue - was marked up quickly, significant bruising appearing under his eye as the pair traded punishing blows.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, matched the intensity with effective uppercuts as the pace rarely dropped over the full distance.
Arthur edged the decision with judges scorecards of even score, 115-113 and wider gap, but both fighters were determined and brave, all the qualities their inspiration Hatton stood for.
