UK light heavyweight Buatsi earned a win yet underwhelming start to his career under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's activity level seemingly secured him a split decision in a competitive 10-round fight at the Co-op Live Arena.
But the challenger had his moments, delivering effective individual punches in a match where the techniques didn't complement each other, with constant grabbing marring the contest.
The officials had it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my best showing but how do champions respond? They prevail," Buatsi commented in the ring as he was faced negative reactions from the spectators.
The Londoner got back to winning after his initial pro loss - an epic battle with his previous opponent in February - to secure a 20th pro success.
Various analysts felt Parker, thirty-one, merited the victory but the Derbyshire fighter endured the second loss of his 28-fight professional journey.
The Champion Claims Triumph in Dull Match
His career development has plateaued in recent years through promotional disputes and considerable time of lack of fights.
Earlier forecast as the future of UK boxing, he now stands trying to realize his potential - maybe in what appears to be a last chance - under fresh management.
He began cautiously, with the two boxers willing to take risks in a tentative initial rounds.
The challenger, whose single earlier setback came at super middleweight against John Ryder in 2022 after breaking his hand during the bout, was participating in his second bout at light-heavyweight.
By the middle rounds, Parker had kept it close, making life awkward for the favorite, who operated with a solid defense to barely take the cleaner stanzas.
The spectators' acceptance decreased amid frequent holding, with Parker going down multiple occasions during clinches.
The underdog ended stronger, but multiple periods were hard to judge. However, ex-champion the boxing analyst - commentating on Dazn - had it strongly leaning for the challenger, awarding Buatsi just a single round.
Buatsi progresses, but with the likes of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - judging by this showing - still looks steep.
Ricky Hatton Homage Inspires All-Manchester Fight
In the supporting bout, Lyndon Arthur defeated Bradley Rea by divided decision in a terrific Manchester vs Manchester battle to secure the European light-heavyweight belt.
The stadium grew hushed for a deeply poignant tribute prior to the bout as 10 bells sounded in honor of Manchester's own Hatton, who succumbed 46 years old in last month.
It was the initial significant broadcast match card in the location since Hatton's passing. As the last bell ended, the stillness gave way to a shout of "there's only one Ricky Hatton".
The action during the bout matched the emotion. Arthur was sent to the canvas by a significant blow directly to the face in the early round, standing uncertainly to his position on the eight count.
But the Manchester United-red equipment of the fighter soon advanced again. On the opposite side, Rea - in rival City blue - was damaged quickly, noticeable damage forming under his face as the pair landed punishing blows.
Rea, twenty-seven, landed power punches but the champion, 34, matched the intensity with stinging uppercuts as the action rarely dropped over the full distance.
Arthur narrowly won the decision with judges scorecards of draw, close margin and clearer margin, but both fighters were persistent and full of heart, exactly what their hero the legend stood for.
