UK light heavyweight Buatsi secured a victory yet underwhelming start to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a split decision in a competitive 10-round contest at the boxing arena.
But Parker had his moments, delivering effective single shots in a contest where the approaches didn't complement each other, with excessive holding spoiling the fight.
Two judges had it 96-94 for to the winner, with the third a even score.
"It wasn't my greatest performance but what do champions do? They win," Buatsi stated in the ringside as he was met by disapproval from the audience.
The Londoner returned to winning ways after his initial pro loss - an thrilling contest with Callum Smith in February - to claim a twentieth professional win.
Several boxing experts believed Parker, 31, should have been awarded the victory but the Derbyshire fighter endured the second loss of his 28-bout professional journey.
Buatsi Secures Triumph in Uneventful Match
His career development has slowed in recent years through contract problems and considerable time of lack of fights.
Previously predicted as the rising talent of British boxing, he now stands trying to achieve expectations - perhaps in what feels like a ultimate opportunity - under fresh management.
He began cautiously, with both competitors prepared 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 damaging his hand mid-fight, was competing in only his second appearance at light heavyweight.
By the halfway stage, the underdog had kept it close, causing problems for Buatsi, who utilized a solid defense to edge the more effective rounds.
The audience's tolerance decreased amid frequent holding, with the challenger slipping to the canvas several times during close exchanges.
Parker ended stronger, but several stanzas were hard to judge. Although, ex-champion the boxing analyst - working with Dazn - had it particularly wide for Parker, awarding Buatsi just one round.
The victor progresses, but with the presence of power puncher Benavidez, and Russian champion Bivol leading the category, the path to championship success - based on this performance - still seems difficult.
Ricky Hatton Tribute Fuels Local Rivalry Contest
In the supporting bout, Lyndon Arthur beat Rea by split-decision in a terrific all-Manchester fight to capture the European light-heavyweight title.
The arena grew hushed for a extremely emotional homage ahead of the contest as 10 bells rang in remembrance of the city's native Ricky Hatton, who succumbed aged 46 in last month.
It was the initial significant broadcast boxing event in the location since the passing of Hatton. As the final chime ended, the stillness transitioned to a chant of "only one Hatton".
The competition during the bout reflected the sentiment. Arthur was knocked down by a powerful punch squarely on the jaw in the initial stage, standing uncertainly to his feet on the referee's count.
But the traditional red shorts of Arthur soon came forward again. From the other corner, Rea - in rival City blue - was damaged quickly, noticeable damage developing under his orbital area as the pair traded powerful shots.
Rea, 27, delivered effective strikes but the champion, thirty-four, matched the intensity with effective uppercuts as the pace hardly diminished over the full distance.
The victor edged the result with official scores of draw, narrow lead and 115-112, but the two athletes were relentless and courageous, all the qualities their idol Hatton represented.
