UK light-heavyweight Buatsi secured a victory yet unimpressive start to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output likely gained him a majority decision in a close 10-round fight at the Co-op Live Arena.
But Parker had his moments, landing some telling clean blows in a match where the styles failed to mesh, with excessive holding disrupting the action.
The scoring judges judged it 96-94 for to the winner, with the remaining official a even score.
"It wasn't my finest performance but what defines champions? They win," the victor said in the arena as he was greeted with negative reactions from the crowd.
The Londoner got back to winning after his first professional defeat - an thrilling contest with Callum Smith in last winter - to secure a 20th career victory.
Various analysts believed Parker, thirty-one, should have been awarded the win but the British boxer endured the second defeat of his 28-fight professional journey.
The Champion Earns Triumph in Slow Encounter
The fighter's advancement has slowed in recent times through promotional disputes and considerable time of inactivity.
Previously predicted as the rising talent of the British fight scene, he now is positioned trying to achieve expectations - possibly in what seems like a last chance - under fresh management.
He began cautiously, with both competitors willing to take risks in a calculated initial rounds.
Parker, whose sole prior loss came at the lower weight class against his former opponent in two years ago after suffering a hand injury in the contest, was participating in his second appearance at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for the favorite, who worked behind a protective stance to barely take the more effective periods.
The crowd's patience decreased because of constant clinching, with the challenger falling to the floor multiple occasions during close exchanges.
Parker closed well, but multiple periods were hard to judge. Although, former world champion the boxing analyst - providing analysis for Dazn - had it particularly wide for the challenger, awarding Buatsi just only one stanza.
Buatsi moves on, but with the caliber of hard hitter Benavidez, and Russian star Bivol dominating the weight class, the path to championship success - judging by this showing - still seems difficult.
Hatton Tribute Inspires Manchester Derby Contest
In the supporting bout, Arthur defeated Bradley Rea by split-decision in a outstanding local rivalry fight to capture the continental belt.
The stadium grew hushed for a profoundly moving homage before the fight as 10 bells rang in honor of local hero Hatton, who died 46 years old in last month.
It was the initial significant broadcast boxing event in the urban center since the passing of Hatton. As the concluding sound disappeared, the stillness gave way to a chant of "only one Hatton".
The fighting during the bout reflected the emotion. The eventual winner was floored by a left hook squarely on the face in the initial stage, getting up unsteadily to his footing on the eight count.
But the Manchester United-red equipment of the fighter soon pressed ahead again. From the other corner, the opponent - in rival City blue - was injured quickly, heavy swelling appearing under his face as the pair landed hard punches.
Rea, 27, delivered effective strikes but Arthur, thirty-four, matched the intensity with effective uppercuts as the intensity hardly diminished over twelve rounds.
Arthur narrowly won the decision with judges scorecards of even score, 115-113 and clearer margin, but both fighters were persistent and brave, everything their inspiration the legend stood for.
