UK light heavyweight Buatsi earned a win yet unimpressive debut to life under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a split decision in a tight 10-round fight at the boxing arena.
But the challenger found success, landing some telling clean blows in a match where the styles failed to mesh, with excessive holding marring the action.
The officials had it 96-94 in favor to Buatsi, with the final judge a even score.
"It wasn't my greatest showing but what defines champions? They succeed," Buatsi said in the ring as he was met by disapproval from the spectators.
The Londoner bounced back after his maiden career setback - an epic battle with his previous opponent in February - to secure a 20th pro success.
Several boxing experts felt the challenger, 31, should have been awarded the victory but the contender experienced the second setback of his 28-fight professional journey.
The Champion Secures Win in Uneventful Encounter
Buatsi's progress has stalled in recent years through contract problems and considerable time of inactivity.
Earlier forecast as the next star of the British fight scene, he now finds himself trying to achieve expectations - maybe in what feels like a final roll of the dice - under recent support.
He started carefully, with neither fighter prepared to take risks in a cagey initial rounds.
Parker, whose only previous defeat came at the lower weight class against John Ryder in 2022 after breaking his hand mid-fight, was making just his second bout at the higher weight class.
By the halfway stage, the underdog had stayed in contention, causing problems for the favorite, who worked behind a solid defense to narrowly win the more effective rounds.
The audience's tolerance decreased amid constant clinching, with Parker slipping to the canvas several times during close exchanges.
Parker ended stronger, but several stanzas were difficult to score. However, ex-champion Carl Frampton - providing analysis for Dazn - had it significantly in favor for Parker, scoring for Buatsi just one round.
The victor progresses, but with the presence of hard hitter David Benavidez, and Russian star Bivol ruling the division, the journey to titles - on this evidence - still seems difficult.
Hatton Memorial Fuels Manchester Derby Contest
In the supporting bout, Arthur beat Bradley Rea by divided decision in a terrific local rivalry fight to secure the European light heavyweight championship.
The venue fell silent for a extremely emotional memorial before the fight as ten bells sounded in memory of Manchester's own Hatton, who passed away 46 years old in last month.
It was the premier televised match card in the location since Hatton's passing. As the last bell disappeared, the silence transitioned to a chant of "only one Ricky Hatton".
The competition within the ring reflected the sentiment. The eventual winner was floored by a left hook directly to the jaw in the early round, rising shakily to his position on the referee's count.
But the red-colored shorts of Arthur soon advanced again. From the other corner, the opponent - in rival City blue - was marked up soon, heavy swelling forming under his orbital area as the pair landed powerful shots.
Rea, twenty-seven, landed power punches but Arthur, 34, responded in kind with powerful upward punches as the action hardly diminished over the full distance.
The victor barely secured the result with judges scorecards of 114-114, narrow lead and 115-112, but both fighters were persistent and courageous, all the qualities their inspiration Hatton stood for.
