UK light heavyweight Buatsi earned a win yet underwhelming debut to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a split decision in a close ten-round fight at the Manchester venue.
But the challenger found success, landing some telling single shots in a fight where the styles failed to mesh, with excessive holding disrupting the contest.
The scoring judges scored it 96-94 to the winner, with the final judge a even score.
"It wasn't my greatest showing but how do champions respond? They prevail," Buatsi said in the ringside as he was greeted with negative reactions from the spectators.
The British fighter bounced back after his maiden career setback - an memorable fight with Callum Smith in last winter - to secure a 20th pro success.
Some ringside observers believed Parker, thirty-one, merited the decision but the Derbyshire fighter experienced the second setback of his 28-bout career.
The Champion Claims Win in Slow Encounter
His career development has plateaued in the past few years through management issues and long spells of inactivity.
Earlier forecast as the next star of UK boxing, he now stands trying to realize his potential - possibly in what feels like a last chance - under fresh management.
He started carefully, with both competitors willing to overcommit in a calculated opening few rounds.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in two years ago after breaking his hand during the bout, was competing in only his second appearance at light-heavyweight.
By the halfway stage, Parker had stayed in contention, making life awkward for the favorite, who utilized a solid defense to barely take the cleaner rounds.
The audience's tolerance wore thin because of frequent holding, with the challenger slipping to the canvas multiple occasions during grappling.
The underdog ended stronger, but many rounds were hard to judge. Although, previous titleholder the boxing analyst - commentating on Dazn - had it particularly wide for the challenger, awarding Buatsi just only one stanza.
Buatsi advances, but with the caliber of power puncher David Benavidez, and Russian champion Bivol leading the category, the road to world glory - judging by this showing - still looks steep.
Hatton Tribute Fuels All-Manchester Fight
In the secondary fight, Arthur beat Rea by divided decision in a terrific Manchester vs Manchester contest to secure the continental belt.
The arena grew hushed for a extremely emotional tribute ahead of the contest as a bell ceremony rang in memory of the city's native Hatton, who succumbed aged 46 in September.
It was the initial significant broadcast match card in the urban center since the passing of Hatton. As the concluding sound disappeared, the silence transitioned to a chant of "only one Hatton".
The fighting inside the ropes reflected the emotion. The eventual winner was floored by a left hook directly to the chin in the early round, getting up unsteadily to his position on the referee's count.
But the traditional red equipment of Arthur soon came forward again. On the opposite side, Rea - in competitive blue - was injured early, heavy swelling appearing under his eye as the duo landed powerful shots.
Rea, 27, landed power punches but the champion, 34, responded in kind with effective uppercuts as the intensity seldom decreased over 12 rounds.
The victor edged the decision with official scores of draw, 115-113 and wider gap, but both fighters were persistent and courageous, everything their hero Hatton stood for.
