British light heavyweight Buatsi earned a win yet disappointing 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 judges' verdict in a tight 10-round fight at the boxing arena.
But the challenger found success, delivering effective individual punches in a match where the techniques didn't complement each other, with frequent clinching disrupting the action.
Two judges judged it 96-94 to the winner, with the third a 95-95.
"It wasn't my greatest display but what do champions do? They prevail," Buatsi commented in the ring as he was met by boos from the audience.
The British fighter bounced back after his maiden career setback - an memorable fight with his previous opponent in February - to claim a twentieth professional win.
Several boxing experts felt Parker, 31, merited the win but the contender experienced the second setback of his 28-fight career.
The Champion Secures Victory in Uneventful Match
The fighter's advancement has slowed in the past few years through contract problems and extended periods of inactivity.
Earlier forecast as the rising talent of UK boxing, he now stands trying to achieve expectations - maybe in what feels like a last chance - under fresh management.
He opened tentatively, with both competitors prepared to be aggressive in a tentative initial rounds.
Parker, whose single earlier setback came at super middleweight against his former opponent in 2022 after breaking his hand mid-fight, was making just his second bout at light heavyweight.
By the halfway stage, the underdog had kept it close, causing problems for Buatsi, who worked behind a solid defense to edge the better stanzas.
The crowd's patience diminished during regular grabbing, with Parker going down several times during close exchanges.
The underdog closed well, but several stanzas were hard to judge. Although, ex-champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for the challenger, giving Buatsi just one round.
The victor advances, but with the likes of power puncher David Benavidez, and Russian champion Bivol dominating the weight class, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Tribute Motivates Local Rivalry Contest
In the co-main event, Lyndon Arthur beat Bradley Rea by close verdict in a outstanding Manchester vs Manchester contest to win the continental title.
The venue grew hushed for a profoundly moving tribute before the fight as a bell ceremony sounded in honor of local hero Hatton, who died aged 46 in last month.
It was the initial significant broadcast match card in the location since Hatton's passing. As the last bell disappeared, the silence gave way to a roar of "there's just one Ricky Hatton".
The competition during the bout matched the feeling. Arthur was knocked down by a left hook directly to the face in the initial stage, getting up unsteadily to his footing on the count of eight.
But the red-colored attire of Arthur soon advanced again. On the opposite side, Rea - in competitive blue - was injured early, noticeable damage appearing under his eye as the fighters landed punishing blows.
The challenger, twenty-seven, connected with strong blows but Arthur, thirty-four, matched the intensity with effective uppercuts as the action rarely dropped over the full distance.
Arthur narrowly won the verdict with official scores of draw, 115-113 and 115-112, but each competitor were determined and brave, all the qualities their inspiration Hatton represented.
