British light heavyweight Buatsi earned a win yet underwhelming beginning to life under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a split decision in a close 10-round fight at the Manchester venue.
But the challenger landed significant shots, connecting with powerful clean blows in a match where the techniques didn't complement each other, with frequent clinching spoiling the action.
The scoring judges judged it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my finest showing but what defines champions? They win," the victor stated in the ringside as he was faced negative reactions from the spectators.
The Londoner got back to winning after his maiden career setback - an thrilling contest with his previous opponent in February - to secure a twentieth professional win.
Some ringside observers believed the challenger, 31, deserved the victory but the Derbyshire fighter experienced the second loss of his 28-fight professional journey.
The Champion Earns Triumph in Slow Encounter
His career development has stalled in the past few years through contract problems and long spells of lack of fights.
Earlier forecast as the future of UK boxing, he now is positioned trying to realize his potential - perhaps in what feels like a final roll of the dice - under recent support.
He opened tentatively, with both competitors ready to be aggressive in a calculated initial rounds.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in two years ago after damaging his hand mid-fight, was participating in his second bout at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for the favorite, who operated with a protective stance to edge the better periods.
The audience's tolerance diminished because of frequent holding, with Parker going down several times during grappling.
The underdog ended stronger, but multiple periods were challenging to assess. Although, former world champion Carl Frampton - working with Dazn - had it particularly wide for Parker, giving Buatsi just a single round.
Buatsi progresses, but with the caliber of power puncher Benavidez, and Eastern European fighter Bivol dominating the weight class, the path to championship success - on this evidence - still seems difficult.
Hatton Tribute Motivates All-Manchester Battle
In the secondary fight, Arthur beat Rea by divided decision in a excellent all-Manchester fight to secure the European light heavyweight title.
The venue fell silent for a deeply poignant memorial prior to the bout as ten bells rang in remembrance of the city's native Ricky Hatton, who died aged 46 in September.
It was the premier televised fight night in the location since the passing of Hatton. As the last bell disappeared, the quiet became a roar of "only one Ricky Hatton".
The competition inside the ropes matched the sentiment. Arthur was floored by a powerful punch flush on the chin in the initial stage, standing uncertainly to his feet on the eight count.
But the Manchester United-red attire of Arthur soon came forward again. From the other corner, the opponent - in rival City blue - was damaged quickly, heavy swelling forming under his eye as the fighters exchanged powerful shots.
The challenger, twenty-seven, delivered effective strikes but Arthur, thirty-four, fought fire with fire with effective uppercuts as the pace hardly diminished over the full distance.
The victor edged the decision with official scores of draw, 115-113 and clearer margin, but both fighters were persistent and brave, all the qualities their inspiration the legend embodied.
