British light-heavyweight Buatsi made a winning yet unimpressive beginning to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a split decision in a close 10-round bout at the boxing arena.
But the challenger had his moments, connecting with powerful clean blows in a match where the styles didn't complement each other, with excessive holding marring the fight.
Two judges had it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my finest showing but how do champions respond? They win," Buatsi commented in the ring as he was faced disapproval from the crowd.
The British fighter bounced back after his first professional defeat - an memorable fight with Callum Smith in February - to secure a twentieth professional win.
Various analysts felt Parker, 31, should have been awarded the decision but the British boxer experienced the second defeat of his 28-bout professional journey.
Buatsi Secures Triumph in Dull Affair
Buatsi's progress has slowed in recent years through contract problems and long spells of inactivity.
Once tipped as the rising talent of the British fight scene, 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 the two boxers ready to be aggressive in a cagey early stages.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in two years ago after suffering a hand injury mid-fight, was participating in his second appearance at light heavyweight.
By the halfway stage, the underdog had made it competitive, making life awkward for the favorite, who utilized a protective stance to edge the better periods.
The audience's tolerance diminished amid regular grabbing, with Parker going down several times during grappling.
The underdog closed well, but several stanzas were hard to judge. Nevertheless, former world champion Carl Frampton - working with Dazn - had it particularly wide for the challenger, awarding Buatsi just one round.
The victor advances, but with the presence of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the journey to titles - on this evidence - still appears challenging.
Hatton Tribute Motivates All-Manchester Contest
In the co-main event, Arthur overcame Rea by divided decision in a terrific all-Manchester contest to win the European light-heavyweight title.
The stadium became quiet for a profoundly moving memorial before the fight as ten bells sounded in remembrance of Manchester's own Hatton, who succumbed aged 46 in September.
It was the first major televised boxing event in the city since Hatton's passing. As the final chime disappeared, the quiet transitioned to a roar of "only one Hatton".
The fighting inside the ropes matched the feeling. The eventual winner was sent to the canvas by a left hook directly to the chin in the initial stage, rising shakily to his footing on the referee's count.
But the Manchester United-red equipment of Arthur soon advanced again. From the other corner, Rea - in competitive blue - was marked up quickly, noticeable damage forming under his face as the fighters exchanged punishing blows.
Rea, twenty-seven, landed power punches but Arthur, thirty-four, matched the intensity with powerful upward punches as the intensity seldom decreased over 12 rounds.
Arthur narrowly won the verdict with official scores of even score, narrow lead and clearer margin, but each competitor were relentless and courageous, all the qualities their idol the legend stood for.
