UK light heavyweight Buatsi earned a win yet unimpressive debut to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a split decision in a tight ten-round bout at the boxing arena.
But the challenger landed significant shots, connecting with powerful clean blows in a fight where the approaches never quite clicked, with constant grabbing marring the contest.
Two judges had it 96-94 for to the winner, with the third a even score.
"It wasn't my finest performance but how do champions respond? They succeed," the victor said in the arena as he was faced disapproval from the audience.
The British fighter returned to winning ways after his first professional defeat - an epic battle with Callum Smith in last winter - to earn a 20th pro success.
Several boxing experts felt Parker, thirty-one, should have been awarded the decision but the British boxer suffered the second setback of his 28-match career.
Buatsi Earns Win in Dull Affair
Buatsi's progress has plateaued in recent years through contract problems and extended periods of inactivity.
Earlier forecast as the rising talent of British boxing, he now is positioned trying to fulfil that early promise - possibly in what appears to be a final roll of the dice - under recent support.
He began cautiously, with neither fighter ready to take risks in a calculated opening few rounds.
The challenger, whose sole prior loss came at super middleweight against his former opponent in two years ago after damaging his hand during the bout, was making just his second bout at light-heavyweight.
By the halfway stage, the underdog had kept it close, causing problems for Buatsi, who utilized a protective stance to barely take the cleaner rounds.
The audience's tolerance wore thin because of regular grabbing, with Parker slipping to the canvas multiple occasions during close exchanges.
The underdog ended stronger, but multiple periods were challenging to assess. Nevertheless, ex-champion Carl Frampton - working with Dazn - had it particularly wide for the challenger, awarding Buatsi just only one stanza.
The victor progresses, but with the caliber of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the path to championship success - judging by this showing - still seems difficult.
Ricky Hatton Tribute Inspires All-Manchester Battle
In the secondary fight, Arthur defeated Rea by split-decision in a excellent all-Manchester contest to capture the European light heavyweight championship.
The venue grew hushed for a profoundly moving tribute ahead of the contest as a bell ceremony tolled in honor of the city's native Hatton, who succumbed 46 years old in recently.
It was the initial significant broadcast match card in the location since the passing of Hatton. As the last bell faded, the stillness gave way to a roar of "there's just one Ricky Hatton".
The fighting inside the ropes equaled the sentiment. The eventual winner was sent to the canvas by a significant blow directly to the jaw in the initial stage, getting up unsteadily to his footing on the count of eight.
But the red-colored attire of Arthur soon advanced again. From the other corner, Rea - in competitive blue - was injured early, noticeable damage forming under his face as the fighters exchanged punishing blows.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, fought fire with fire with stinging uppercuts as the pace rarely dropped over twelve rounds.
The victor barely secured the result with official scores of even score, close margin and wider gap, but both fighters were persistent and courageous, all the qualities their inspiration the legend stood for.
