UK light-heavyweight Buatsi earned a win yet unimpressive debut to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level likely gained him a judges' verdict in a tight 10-round bout at the boxing arena.
But the challenger found success, connecting with powerful clean blows in a contest where the techniques never quite clicked, with excessive holding marring the contest.
The officials scored it 96-94 in favor to the winner, with the final judge a even score.
"It wasn't my best showing but how do champions respond? They prevail," Buatsi said in the ringside as he was met by negative reactions from the crowd.
The Londoner bounced back after his first professional defeat - an memorable fight with his previous opponent in last winter - to secure a 20th pro success.
Various analysts thought Parker, thirty-one, merited the decision but the contender experienced the second defeat of his 28-bout professional journey.
The Champion Secures Win in Slow Encounter
His career development has plateaued in the past few years through management issues and considerable time of lack of fights.
Earlier forecast as the next star of British boxing, he now is positioned trying to realize his potential - perhaps in what appears to be a final roll of the dice - under recent support.
He opened tentatively, with neither fighter prepared to be aggressive in a calculated early stages.
The challenger, whose only previous defeat came at super middleweight against John Ryder in 2022 after damaging his hand mid-fight, was making just his second bout at the higher weight class.
By the fight's midpoint, the underdog had kept it close, causing problems for Buatsi, who operated with a protective stance to edge the better rounds.
The crowd's patience decreased amid regular grabbing, with the challenger going down several times during clinches.
Parker finished the stronger, but several stanzas were challenging to assess. Although, previous titleholder Carl Frampton - commentating on Dazn - had it particularly wide for Parker, awarding Buatsi just a single round.
Buatsi moves on, but with the caliber of power puncher David Benavidez, and Eastern European fighter Bivol ruling the division, the road to world glory - on this evidence - still seems difficult.
Hatton Tribute Motivates All-Manchester Contest
In the supporting bout, Arthur overcame Rea by split-decision in a excellent local rivalry battle to capture the European light heavyweight championship.
The stadium grew hushed for a deeply poignant tribute before the fight as a bell ceremony sounded in memory of local hero Hatton, who passed away at age 46 in September.
It was the premier televised boxing event in the location since Hatton's passing. As the final chime ended, the quiet gave way to a shout of "there's only one Hatton".
The fighting inside the ropes matched the sentiment. Arthur was sent to the canvas by a significant blow directly to the jaw in the second round, rising shakily to his feet on the referee's count.
But the traditional red equipment of Arthur soon came forward again. On the opposite side, Rea - in rival City blue - was injured soon, heavy swelling appearing under his face as the duo exchanged punishing blows.
The challenger, 27, delivered effective strikes but Arthur, 34, fought fire with fire with effective uppercuts as the action hardly diminished over twelve rounds.
The victor narrowly won the result with judges scorecards of draw, 115-113 and 115-112, but each competitor were determined and full of heart, all the qualities their inspiration the legend embodied.
