UK light heavyweight Buatsi secured a victory yet underwhelming debut to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output appeared to earn him a split decision in a tight 10-round bout at the boxing arena.
But Parker landed significant shots, connecting with powerful individual punches in a match where the approaches failed to mesh, with constant grabbing disrupting the contest.
Two judges scored it 96-94 for to Buatsi, with the third a even score.
"It wasn't my greatest performance but what do champions do? They succeed," Buatsi stated in the ringside as he was faced negative reactions from the audience.
The British fighter bounced back after his initial pro loss - an epic battle with Callum Smith in February - to secure a twentieth professional win.
Some ringside observers felt the challenger, 31, merited the win but the contender experienced the second setback of his 28-bout career.
The Champion Earns Triumph in Slow Affair
Buatsi's progress has plateaued in recent years through promotional disputes and long spells of lack of fights.
Earlier forecast as the next star of UK boxing, he now finds himself trying to achieve expectations - possibly in what feels like a final roll of the dice - under fresh management.
He started carefully, with neither fighter ready to take risks in a tentative initial rounds.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in 2022 after suffering a hand injury mid-fight, was participating in his second bout at light heavyweight.
By the middle rounds, Parker had kept it close, making life awkward for the favorite, who utilized a protective stance to narrowly win the cleaner stanzas.
The spectators' acceptance diminished amid frequent holding, with the challenger going down multiple occasions during grappling.
Parker closed well, but multiple periods were hard to judge. Although, ex-champion Carl Frampton - commentating on the broadcast network - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
Buatsi moves on, but with the caliber of power puncher David Benavidez, and Russian star Dmitry Bivol ruling the division, the road to world glory - based on this performance - still seems difficult.
Hatton Homage Motivates Local Rivalry Contest
In the co-main event, Lyndon Arthur overcame Bradley Rea by split-decision in a outstanding local rivalry contest to win the European light-heavyweight championship.
The venue grew hushed for a profoundly moving homage prior to the bout as ten bells tolled in memory of local hero Ricky Hatton, who succumbed at age 46 in September.
It was the initial significant broadcast fight night in the urban center since Hatton's passing. As the final chime disappeared, the silence became a shout of "there's just one Hatton".
The competition during the bout reflected the emotion. The eventual winner was floored by a powerful punch flush on the chin in the early round, standing uncertainly to his footing on the count of eight.
But the Manchester United-red equipment of the fighter soon came forward again. On the opposite side, the opponent - in competitive blue - was marked up quickly, significant bruising developing under his face as the duo traded hard punches.
Rea, 27, connected with strong blows but Arthur, 34, responded in kind with powerful upward punches as the action seldom decreased over twelve rounds.
The victor barely secured the verdict with official scores of even score, close margin and 115-112, but the two athletes were relentless and courageous, all the qualities their hero Hatton stood for.
