British light heavyweight Buatsi secured a victory yet disappointing beginning to life under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a judges' verdict in a competitive ten-round contest at the Co-op Live Arena.
But Parker had his moments, landing some telling single shots in a contest where the techniques didn't complement each other, with constant grabbing disrupting the contest.
Two judges scored it 96-94 for to Buatsi, with the third a 95-95.
"It wasn't my finest performance but what defines champions? They succeed," the victor stated in the ring as he was faced negative reactions from the spectators.
The Londoner bounced back after his first professional defeat - an epic battle with his previous opponent in last winter - to secure a 20th pro success.
Some ringside observers believed the challenger, 31, should have been awarded the victory but the British boxer suffered the second defeat of his 28-match career.
Buatsi Secures Win in Slow Encounter
The fighter's advancement has stalled in recent times through promotional disputes and considerable time of lack of fights.
Once tipped as the next star of UK boxing, he now stands trying to achieve expectations - possibly in what feels like a ultimate opportunity - under recent support.
He opened tentatively, with both competitors willing to take risks in a cagey opening few rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in 2022 after breaking his hand mid-fight, was making just his second bout at the higher weight class.
By the fight's midpoint, the underdog had stayed in contention, causing problems for the favorite, who worked behind a tight guard to narrowly win the more effective periods.
The crowd's patience diminished amid frequent holding, with the challenger slipping to the canvas several times during clinches.
Parker ended stronger, but many rounds were hard to judge. However, ex-champion Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, awarding Buatsi just only one stanza.
Buatsi progresses, but with the caliber of heavy hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Memorial Inspires Manchester Derby Contest
In the supporting bout, Lyndon Arthur overcame Bradley Rea by close verdict in a outstanding Manchester vs Manchester battle to secure the European light heavyweight belt.
The arena grew hushed for a extremely emotional memorial ahead of the contest as a bell ceremony sounded in honor of Manchester's own Hatton, who succumbed aged 46 in September.
It was the initial significant broadcast fight night in the urban center since the passing of Hatton. As the final chime ended, the silence became a chant of "only one Hatton".
The competition within the ring matched the emotion. Arthur was floored by a significant blow flush on the chin in the initial stage, getting up unsteadily to his feet on the referee's count.
But the red-colored attire of the fighter soon pressed ahead again. Across the ring, the opponent - in opposing blue - was injured soon, significant bruising developing under his orbital area as the pair traded punishing blows.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, matched the intensity with powerful upward punches as the pace seldom decreased over twelve rounds.
The victor edged the verdict with scoring tallies of even score, 115-113 and wider gap, but each competitor were relentless and brave, all the qualities their idol Hatton stood for.
