UK light heavyweight Buatsi made a winning yet disappointing debut to his career under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a majority decision in a tight 10-round contest at the boxing arena.
But Parker had his moments, connecting with powerful individual punches in a fight where the styles didn't complement each other, with excessive holding disrupting the contest.
The officials scored it 96-94 in favor to Buatsi, with the final judge a 95-95.
"It wasn't my best performance but what do champions do? They win," the victor commented in the arena as he was met by negative reactions from the spectators.
The British fighter bounced back after his first professional defeat - an epic battle with Callum Smith in last winter - to secure a 20th career victory.
Some ringside observers believed Parker, thirty-one, should have been awarded the win but the British boxer endured the second setback of his 28-bout professional journey.
Buatsi Earns Win in Uneventful Affair
The fighter's advancement has slowed in recent times through management issues and long spells of lack of fights.
Previously predicted as the next star of UK boxing, he now is positioned trying to realize his potential - maybe in what seems like a last chance - under recent support.
He opened tentatively, with both competitors willing to be aggressive in a calculated opening few rounds.
Parker, whose single earlier setback came at super-middleweight against John Ryder in 2022 after breaking his hand during the bout, was making just his second bout at light-heavyweight.
By the halfway stage, the underdog had made it competitive, making life awkward for Buatsi, who operated with a protective stance to narrowly win the better rounds.
The audience's tolerance decreased because of constant clinching, with the challenger falling to the floor various instances during close exchanges.
Parker finished the stronger, but several stanzas were challenging to assess. Although, ex-champion Carl Frampton - commentating on Dazn - had it strongly leaning for the challenger, giving Buatsi just a single round.
The victor advances, but with the presence of heavy hitter David Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Tribute Inspires Local Rivalry Contest
In the co-main event, Lyndon Arthur beat Bradley Rea by split-decision in a excellent Manchester vs Manchester battle to capture the European light-heavyweight championship.
The stadium became quiet for a extremely emotional tribute prior to the bout as 10 bells tolled in honor of local hero Hatton, who died 46 years old in September.
It was the initial significant broadcast fight night in the city since Hatton's passing. As the concluding sound disappeared, the stillness transitioned to a shout of "there's just one Hatton".
The competition inside the ropes reflected the feeling. Arthur was knocked down by a left hook directly to the chin in the early round, getting up unsteadily to his feet on the count of eight.
But the traditional red shorts of the fighter soon advanced again. Across the ring, the opponent - in competitive blue - was injured soon, heavy swelling developing under his face as the pair traded punishing blows.
The challenger, 27, delivered effective strikes but Arthur, 34, matched the intensity with stinging uppercuts as the pace rarely dropped over 12 rounds.
The victor narrowly won the result with scoring tallies of even score, 115-113 and 115-112, but both fighters were persistent and courageous, exactly what their idol the legend stood for.
