UK light-heavyweight Buatsi made a winning yet underwhelming start to his career under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a majority decision in a close ten-round fight at the boxing arena.
But Parker landed significant shots, delivering effective clean blows 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 third a 95-95.
"It wasn't my best showing but how do champions respond? They prevail," Buatsi said in the ring as he was faced negative reactions from the audience.
The Londoner bounced back after his first professional defeat - an memorable fight with his previous opponent in February - to earn a twentieth professional win.
Various analysts thought Parker, thirty-one, should have been awarded the decision but the British boxer experienced the second loss of his 28-fight career.
The Champion Earns Triumph in Dull Affair
The fighter's advancement has plateaued in the past few years through management issues and long spells of lack of fights.
Once tipped as the rising talent of UK boxing, he now finds himself trying to realize his potential - perhaps in what seems like a last chance - under recent support.
He began cautiously, with neither fighter prepared to overcommit in a cagey opening few rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after damaging his hand mid-fight, was competing in only his second appearance at the higher weight class.
By the middle rounds, the underdog had made it competitive, making life awkward for the favorite, who worked behind a protective stance to barely take the cleaner stanzas.
The audience's tolerance wore thin amid regular grabbing, with Parker going down various instances during close exchanges.
Parker closed well, but many rounds were difficult to score. Nevertheless, ex-champion the boxing analyst - working with Dazn - had it significantly in favor for Parker, scoring for Buatsi just a single round.
The victor advances, but with the likes of hard hitter Benavidez, and Eastern European fighter Bivol leading the category, the path to championship success - on this evidence - still seems difficult.
Hatton Memorial Motivates Local Rivalry Contest
In the co-main event, Arthur overcame Rea by divided decision in a outstanding local rivalry fight to win the European light-heavyweight belt.
The venue fell silent for a profoundly moving homage ahead of the contest as ten bells tolled in honor of local hero Ricky Hatton, who passed away 46 years old in last month.
It was the initial significant broadcast fight night in the urban center since Hatton's passing. As the last bell ended, the silence gave way to a roar of "only one Ricky Hatton".
The fighting during the bout reflected the emotion. Arthur was floored by a significant blow squarely on the chin in the initial stage, rising shakily to his feet on the count of eight.
But the traditional red equipment of the fighter soon pressed ahead again. Across the ring, Rea - in opposing blue - was injured early, noticeable damage developing under his eye as the fighters traded hard punches.
The challenger, 27, delivered effective strikes but Arthur, thirty-four, responded in kind with stinging uppercuts as the action seldom decreased over 12 rounds.
Arthur barely secured the decision with judges scorecards of even score, 115-113 and clearer margin, but the two athletes were determined and brave, exactly what their inspiration Hatton stood for.
