British light heavyweight Buatsi earned a win yet disappointing start to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a split decision in a competitive ten-round contest at the Co-op Live Arena.
But Parker had his moments, landing some telling individual punches in a contest where the approaches never quite clicked, with excessive holding marring the action.
The officials scored it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my greatest display but what defines champions? They succeed," the victor commented in the ringside as he was greeted with boos from the audience.
The British fighter bounced back after his initial pro loss - an thrilling contest with Callum Smith in February - to earn a twentieth professional win.
Various analysts felt Parker, thirty-one, deserved the decision but the Derbyshire fighter experienced the second loss of his 28-bout career.
The Champion Claims Win in Uneventful Affair
The fighter's advancement has stalled in recent years through contract problems and considerable time of lack of fights.
Once tipped as the future of British boxing, he now is positioned trying to achieve expectations - perhaps in what feels like a ultimate opportunity - under recent support.
He opened tentatively, with both competitors willing to take risks in a calculated opening few rounds.
Parker, whose sole prior loss came at the lower weight class against his former opponent in 2022 after suffering a hand injury in the contest, was making just his second bout at light-heavyweight.
By the halfway stage, the underdog had kept it close, creating difficulties for the favorite, who utilized a tight guard to edge the cleaner stanzas.
The audience's tolerance diminished during regular grabbing, with the challenger slipping to the canvas various instances during close exchanges.
The underdog finished the stronger, but many rounds were challenging to assess. Although, former world champion Carl Frampton - providing analysis for the broadcast network - had it particularly wide for the challenger, scoring for Buatsi just only one stanza.
Buatsi moves on, but with the caliber of power puncher David Benavidez, and Russian champion Dmitry Bivol ruling the division, the path to championship success - on this evidence - still seems difficult.
Hatton Tribute Motivates Manchester Derby Contest
In the supporting bout, Lyndon Arthur beat Rea by close verdict in a excellent Manchester vs Manchester contest to win the European light-heavyweight belt.
The venue grew hushed for a deeply poignant homage ahead of the contest as 10 bells tolled in honor of local hero Ricky Hatton, who passed away aged 46 in last month.
It was the first major televised fight night in the city since Hatton's passing. As the final chime faded, the stillness transitioned to a roar of "there's only one Hatton".
The action inside the ropes reflected the sentiment. Arthur was sent to the canvas by a significant blow squarely on the jaw in the initial stage, getting up unsteadily to his feet on the eight count.
But the red-colored equipment of Arthur soon came forward again. On the opposite side, the opponent - in competitive blue - was marked up soon, heavy swelling developing under his face as the duo exchanged powerful shots.
The challenger, 27, connected with strong blows but the champion, 34, matched the intensity with stinging uppercuts as the pace seldom decreased over twelve rounds.
The victor edged the decision with scoring tallies of even score, close margin and wider gap, but the two athletes were persistent and brave, all the qualities their inspiration the legend represented.
