British light-heavyweight Buatsi secured a victory yet disappointing beginning to life under new promoters with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a judges' verdict in a close 10-round fight at the boxing arena.
But the challenger landed significant shots, connecting with powerful clean blows in a match where the approaches failed to mesh, with frequent clinching marring the action.
The scoring judges scored it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my greatest performance but what do champions do? They succeed," Buatsi stated in the ringside as he was greeted with boos from the spectators.
The Londoner returned to winning ways after his first professional defeat - an epic battle with his previous opponent in February - to earn a twentieth professional win.
Some ringside observers thought Parker, 31, should have been awarded the victory but the Derbyshire fighter experienced the second loss of his 28-fight career.
Buatsi Earns Triumph in Uneventful Match
Buatsi's progress has stalled in the past few years through contract problems and long spells of inactivity.
Previously predicted as the rising talent of the British fight scene, he now finds himself trying to fulfil that early promise - possibly in what seems like a ultimate opportunity - under recent support.
He started carefully, with neither fighter willing to be aggressive in a cagey opening few rounds.
Parker, whose single earlier setback came at the lower weight class against his former opponent in 2022 after breaking his hand during the bout, was making just his second bout at light-heavyweight.
By the middle rounds, the underdog had made it competitive, creating difficulties for the favorite, who worked behind a protective stance to barely take the more effective rounds.
The audience's tolerance diminished amid constant clinching, with the challenger going down multiple occasions during clinches.
Parker finished the stronger, but many rounds were challenging to assess. Nevertheless, previous titleholder the boxing analyst - working with the broadcast network - had it strongly leaning for Parker, giving Buatsi just only one stanza.
Buatsi progresses, but with the presence of power puncher David Benavidez, and Russian champion Bivol dominating the weight class, the road to world glory - judging by this showing - still looks steep.
Hatton Homage Inspires Manchester Derby Contest
In the co-main event, Lyndon Arthur defeated Rea by divided decision in a excellent Manchester vs Manchester fight to win the European light-heavyweight belt.
The venue grew hushed for a extremely emotional homage prior to the bout as 10 bells sounded in honor of Manchester's own Ricky Hatton, who succumbed aged 46 in September.
It was the first major televised boxing event in the city since Hatton's passing. As the last bell disappeared, the quiet transitioned to a chant of "there's only one Hatton".
The action inside the ropes reflected the feeling. The eventual winner was floored by a powerful punch directly to the jaw in the second round, standing uncertainly to his position on the referee's count.
But the red-colored attire of Arthur soon pressed ahead again. On the opposite side, the opponent - in opposing blue - was damaged early, heavy swelling developing under his orbital area as the fighters traded punishing blows.
Rea, 27, delivered effective strikes but Arthur, 34, fought fire with fire with stinging uppercuts as the pace hardly diminished over the full distance.
The victor edged the result with judges scorecards of draw, narrow lead and wider gap, but both fighters were persistent and brave, all the qualities their hero the legend stood for.
