British light-heavyweight Buatsi secured a victory yet unimpressive start to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level likely gained him a split decision in a close 10-round contest at the Manchester venue.
But the challenger found success, landing some telling individual punches in a contest where the styles didn't complement each other, with constant grabbing marring the fight.
The officials judged it 96-94 in favor to the winner, with the third a 95-95 draw.
"It wasn't my greatest display but what do champions do? They succeed," Buatsi commented in the ring as he was met by negative reactions from the crowd.
The British fighter got back to winning after his first professional defeat - an epic battle with Callum Smith in February - to secure a 20th career victory.
Some ringside observers thought Parker, 31, merited the win but the Derbyshire fighter endured the second setback of his 28-fight career.
Buatsi Earns Win in Dull Encounter
His career development has stalled in recent years through promotional disputes and extended periods of inactivity.
Once tipped as the next star of the British fight scene, he now finds himself trying to realize his potential - possibly in what appears to be a ultimate opportunity - under new promotional backing.
He began cautiously, with neither fighter willing to be aggressive in a cagey opening few rounds.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in 2022 after suffering a hand injury in the contest, was participating in his second fight at light-heavyweight.
By the halfway stage, the underdog had kept it close, making life awkward for the favorite, who utilized a solid defense to edge the better periods.
The audience's tolerance diminished amid constant clinching, with Parker going down multiple occasions during clinches.
Parker closed well, but several stanzas were challenging to assess. Nevertheless, ex-champion Carl Frampton - providing analysis for Dazn - had it strongly leaning for the challenger, scoring for Buatsi just a single round.
Buatsi progresses, but with the presence of heavy hitter Benavidez, and Russian star Bivol leading the category, the road to world glory - based on this performance - still seems difficult.
Ricky Hatton Memorial Inspires Local Rivalry Battle
In the supporting bout, Arthur defeated Rea by divided decision in a terrific Manchester vs Manchester battle to capture the continental championship.
The venue fell silent for a profoundly moving memorial ahead of the contest as 10 bells rang in memory of local hero Ricky Hatton, who succumbed at age 46 in last month.
It was the initial significant broadcast boxing event in the city since the passing of Hatton. As the last bell faded, the quiet became a roar of "there's only one Ricky Hatton".
The competition within the ring matched the sentiment. The eventual winner was floored by a powerful punch flush on the jaw in the initial stage, standing uncertainly to his feet on the eight count.
But the red-colored attire of Arthur soon pressed ahead again. From the other corner, the opponent - in competitive blue - was marked up soon, heavy swelling appearing under his face as the fighters traded powerful shots.
The challenger, twenty-seven, connected with strong blows but the champion, 34, matched the intensity with effective uppercuts as the action seldom decreased over the full distance.
Arthur edged the decision with official scores of draw, narrow lead and 115-112, but each competitor were persistent and courageous, everything their hero Hatton embodied.
