UK light heavyweight Buatsi secured a victory yet unimpressive beginning to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a majority decision in a close 10-round bout at the Manchester venue.
But the challenger found success, connecting with powerful individual punches in a match where the styles failed to mesh, with constant grabbing disrupting the fight.
The officials had it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my best display but what defines champions? They succeed," Buatsi stated in the ring as he was met by negative reactions from the crowd.
The Londoner got back to winning after his maiden career setback - an memorable fight with his previous opponent in last winter - to secure a 20th career victory.
Various analysts felt Parker, thirty-one, should have been awarded the win but the contender endured the second defeat of his 28-bout professional journey.
The Champion Secures Triumph in Slow Affair
Buatsi's progress has plateaued in recent years through management issues and considerable time of inactivity.
Earlier forecast as the rising talent of British boxing, he now is positioned trying to realize his potential - perhaps in what appears to be a last chance - under recent support.
He opened tentatively, with the two boxers prepared to be aggressive in a cagey opening few rounds.
The challenger, whose sole prior loss came at super middleweight against his former opponent in two years ago after suffering a hand injury in the contest, was making just his second fight at light heavyweight.
By the halfway stage, the underdog had stayed in contention, making life awkward for the favorite, who operated with a solid defense to barely take the cleaner stanzas.
The spectators' acceptance diminished because of frequent holding, with the challenger going down multiple occasions during clinches.
Parker finished the stronger, but several stanzas were hard to judge. Although, previous titleholder Carl Frampton - working with Dazn - had it strongly leaning for Parker, awarding Buatsi just one round.
Buatsi progresses, but with the caliber of power puncher Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Homage Motivates All-Manchester Contest
In the secondary fight, Arthur beat Rea by split-decision in a outstanding local rivalry battle to secure the European light-heavyweight title.
The venue fell silent for a extremely emotional homage before the fight as a bell ceremony sounded in remembrance of the city's native Ricky Hatton, who died aged 46 in September.
It was the premier televised fight night in the city since the passing of Hatton. As the final chime ended, the quiet became a roar of "there's just one Hatton".
The competition inside the ropes equaled the sentiment. The eventual winner was floored by a significant blow directly to the face in the early round, standing uncertainly to his footing on the eight count.
But the Manchester United-red equipment of Arthur soon pressed ahead again. From the other corner, the opponent - in rival City blue - was damaged soon, noticeable damage appearing under his orbital area as the pair exchanged hard punches.
The challenger, twenty-seven, landed power punches but Arthur, 34, fought fire with fire with stinging uppercuts as the action rarely dropped over the full distance.
Arthur barely secured the result with judges scorecards of draw, 115-113 and 115-112, but the two athletes were persistent and courageous, exactly what their idol Hatton stood for.
