UK light-heavyweight Buatsi secured a victory yet unimpressive beginning to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a judges' verdict in a close 10-round fight at the Manchester venue.
But Parker found success, delivering effective individual punches in a fight where the techniques didn't complement each other, with excessive holding spoiling the contest.
The scoring judges had it 96-94 to Buatsi, with the third a even score.
"It wasn't my greatest showing but what defines champions? They succeed," the victor said in the ringside as he was faced disapproval from the crowd.
The Londoner got back to winning after his initial pro loss - an memorable fight with Callum Smith in last winter - to claim a twentieth professional win.
Various analysts believed the challenger, thirty-one, should have been awarded the victory but the Derbyshire fighter endured the second loss of his 28-bout career.
Buatsi Earns Victory in Dull Encounter
His career development has slowed in recent years through management issues and long spells of inactivity.
Once tipped as the rising talent of UK boxing, he now stands trying to achieve expectations - maybe in what feels like a final roll of the dice - under fresh management.
He opened tentatively, with the two boxers ready to be aggressive in a tentative initial rounds.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in 2022 after breaking his hand mid-fight, was participating in his second bout at the higher weight class.
By the halfway stage, Parker had kept it close, creating difficulties for Buatsi, who operated with a tight guard to barely take the better periods.
The spectators' acceptance diminished because of regular grabbing, with Parker slipping to the canvas multiple occasions during grappling.
Parker closed well, but multiple periods were hard to judge. Although, former world champion the boxing analyst - providing analysis for Dazn - had it significantly in favor for Parker, awarding Buatsi just only one stanza.
The victor advances, but with the presence of power puncher Benavidez, and Russian champion Bivol ruling the division, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Homage Inspires All-Manchester Battle
In the co-main event, Arthur overcame Rea by split-decision in a terrific Manchester vs Manchester battle to win the European light-heavyweight championship.
The arena became quiet for a profoundly moving homage prior to the bout as 10 bells tolled in remembrance of local hero Ricky Hatton, who succumbed aged 46 in recently.
It was the initial significant broadcast boxing event in the urban center since Hatton's passing. As the final chime disappeared, the stillness became a chant of "there's only one Hatton".
The competition within the ring equaled the sentiment. Arthur was floored by a left hook directly to the chin in the second round, getting up unsteadily to his feet on the referee's count.
But the red-colored equipment of Arthur soon came forward again. On the opposite side, the opponent - in competitive blue - was injured early, heavy swelling developing under his orbital area as the fighters traded punishing blows.
The challenger, twenty-seven, landed power punches but the champion, 34, matched the intensity with stinging uppercuts as the intensity seldom decreased over the full distance.
The victor narrowly won the result with judges scorecards of even score, narrow lead and clearer margin, but each competitor were persistent and brave, all the qualities their idol the legend embodied.
