British light-heavyweight Buatsi made a winning yet unimpressive debut to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output likely gained him a split decision in a competitive ten-round bout at the Co-op Live Arena.
But Parker had his moments, connecting with powerful single shots in a match where the approaches failed to mesh, with constant grabbing disrupting the fight.
Two judges had it 96-94 for to the winner, with the third a even score.
"It wasn't my finest showing but how do champions respond? They succeed," the victor commented in the ring as he was met by boos from the spectators.
The British fighter bounced back after his first professional defeat - an epic battle with Callum Smith in February - to claim a 20th career victory.
Several boxing experts thought Parker, thirty-one, should have been awarded the decision but the Derbyshire fighter endured the second setback of his 28-match professional journey.
Buatsi Claims Triumph in Slow Match
His career development has slowed in recent years through promotional disputes and long spells of lack of fights.
Once tipped as the future of British boxing, he now finds himself trying to achieve expectations - perhaps in what appears to be a final roll of the dice - under recent support.
He started carefully, with neither fighter willing to take risks in a calculated initial rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in two years ago after damaging his hand in the contest, was competing in only his second appearance at light-heavyweight.
By the middle rounds, Parker had stayed in contention, causing problems for the favorite, who operated with a tight guard to barely take the better stanzas.
The crowd's patience diminished amid constant clinching, with the challenger falling to the floor several times during grappling.
The underdog closed well, but many rounds were hard to judge. Although, previous titleholder Carl Frampton - commentating on the broadcast network - had it significantly in favor for Parker, awarding Buatsi just one round.
Buatsi advances, but with the likes of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the road to world glory - based on this performance - still appears challenging.
Ricky Hatton Memorial Fuels All-Manchester Battle
In the secondary fight, Arthur beat Bradley Rea by split-decision in a outstanding all-Manchester contest to secure the European light-heavyweight belt.
The venue grew hushed for a profoundly moving homage prior to the bout as ten bells rang in remembrance of local hero Ricky Hatton, who succumbed 46 years old in September.
It was the initial significant broadcast boxing event in the city since the passing of Hatton. As the final chime disappeared, the silence transitioned to a roar of "there's just one Ricky Hatton".
The competition inside the ropes equaled the emotion. The eventual winner was floored by a significant blow directly to the jaw in the second round, getting up unsteadily to his footing on the count of eight.
But the Manchester United-red shorts of the fighter soon advanced again. On the opposite side, Rea - in opposing blue - was damaged early, significant bruising appearing under his face as the fighters traded powerful shots.
The challenger, twenty-seven, connected with strong blows but Arthur, thirty-four, responded in kind with effective uppercuts as the action seldom decreased over twelve rounds.
Arthur barely secured the decision with official scores of 114-114, narrow lead and 115-112, but each competitor were persistent and brave, everything their inspiration the legend represented.
