UK light heavyweight Buatsi earned a win yet unimpressive beginning to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a split decision in a close ten-round bout at the boxing arena.
But the challenger landed significant shots, delivering effective clean blows in a contest where the techniques failed to mesh, with excessive holding marring the action.
Two judges scored it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my finest performance but what do champions do? They succeed," the victor stated in the arena as he was met by negative reactions from the crowd.
The Londoner returned to winning ways after his initial pro loss - an epic battle with Callum Smith in February - to earn a 20th career victory.
Some ringside observers thought Parker, thirty-one, should have been awarded the win but the contender suffered the second setback of his 28-fight professional journey.
Buatsi Earns Victory in Uneventful Encounter
His career development has plateaued in recent times through promotional disputes and considerable time of inactivity.
Once tipped as the rising talent of UK boxing, he now is positioned trying to achieve expectations - maybe in what feels like a last chance - under new promotional backing.
He began cautiously, with neither fighter ready to overcommit in a tentative early stages.
Parker, whose only previous defeat came at super middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was making just his second fight at the higher weight class.
By the halfway stage, Parker had made it competitive, making life awkward for Buatsi, who operated with a solid defense to barely take the better periods.
The audience's tolerance diminished amid frequent holding, with the challenger going down multiple occasions during clinches.
Parker closed well, but many rounds were challenging to assess. Nevertheless, ex-champion the boxing analyst - providing analysis for Dazn - had it particularly wide for Parker, awarding Buatsi just a single round.
The victor progresses, but with the likes of hard hitter Benavidez, and Eastern European fighter Bivol ruling the division, the path to championship success - judging by this showing - still looks steep.
Hatton Memorial Fuels Manchester Derby Fight
In the co-main event, Lyndon Arthur defeated Rea by divided decision in a terrific all-Manchester contest to capture the continental belt.
The stadium became quiet for a deeply poignant homage ahead of the contest as ten bells sounded in honor of the city's native Hatton, who passed away at age 46 in recently.
It was the initial significant broadcast match card in the city since Hatton's passing. As the last bell ended, the stillness transitioned to a chant of "there's just one Ricky Hatton".
The competition inside the ropes matched the sentiment. Arthur was sent to the canvas by a significant blow flush on the face in the early round, rising shakily to his feet on the eight count.
But the red-colored shorts of Arthur soon pressed ahead again. From the other corner, Rea - in rival City blue - was marked up soon, significant bruising appearing under his eye as the fighters traded punishing blows.
Rea, 27, delivered effective strikes but the champion, thirty-four, responded in kind with powerful upward punches as the intensity rarely dropped over the full distance.
The victor narrowly won the result with official scores of even score, 115-113 and clearer margin, but each competitor were persistent and full of heart, all the qualities their idol Hatton represented.
