British light-heavyweight Buatsi earned a win yet disappointing beginning to life under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a majority decision in a competitive 10-round contest at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective individual punches in a fight where the techniques never quite clicked, with excessive holding spoiling the contest.
The officials judged it 96-94 for to the winner, with the remaining official a even score.
"It wasn't my best showing but what defines champions? They win," the victor commented in the arena as he was greeted with disapproval from the audience.
The British fighter bounced back after his initial pro loss - an memorable fight with Callum Smith in February - to earn a 20th career victory.
Some ringside observers thought Parker, 31, merited the win but the contender endured the second defeat of his 28-fight professional journey.
Buatsi Earns Win in Dull Affair
His career development has stalled in the past few years through management issues and long spells of lack of fights.
Earlier forecast as the next star of the British fight scene, he now is positioned trying to achieve expectations - maybe in what appears to be a ultimate opportunity - under fresh management.
He opened tentatively, with the two boxers ready to be aggressive in a tentative early stages.
Parker, whose sole prior loss came at the lower weight class against John Ryder in 2022 after damaging his hand in the contest, was participating in his second appearance at light heavyweight.
By the halfway stage, Parker had kept it close, making life awkward for Buatsi, who worked behind a tight guard to edge the cleaner periods.
The crowd's patience decreased during regular grabbing, with the challenger falling to the floor several times during clinches.
The underdog ended stronger, but several stanzas were difficult to score. Nevertheless, ex-champion Carl Frampton - working with Dazn - had it particularly wide for Parker, scoring for Buatsi just only one stanza.
The victor advances, but with the likes of hard hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Homage Fuels Local Rivalry Battle
In the co-main event, Lyndon Arthur defeated Rea by split-decision in a excellent Manchester vs Manchester fight to secure the European light heavyweight title.
The stadium became quiet for a deeply poignant homage prior to the bout as ten bells sounded in honor of Manchester's own Ricky Hatton, who succumbed at age 46 in recently.
It was the first major televised boxing event in the location since the passing of Hatton. As the concluding sound faded, the stillness transitioned to a chant of "there's just one Ricky Hatton".
The competition within the ring reflected the feeling. Arthur was sent to the canvas by a significant blow flush on the jaw in the early round, rising shakily to his footing on the eight count.
But the Manchester United-red equipment of Arthur soon came forward again. On the opposite side, the opponent - in rival City blue - was injured quickly, noticeable damage developing under his face as the duo traded punishing blows.
Rea, twenty-seven, landed power punches but the champion, 34, matched the intensity with powerful upward punches as the action rarely dropped over 12 rounds.
Arthur edged the result with official scores of even score, 115-113 and clearer margin, but each competitor were relentless and full of heart, all the qualities their idol Hatton embodied.
