UK light heavyweight Buatsi secured a victory yet disappointing beginning to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output likely gained him a majority decision in a competitive 10-round contest at the boxing arena.
But Parker landed significant shots, delivering effective individual punches in a contest where the techniques failed to mesh, with excessive holding spoiling the fight.
The officials scored it 96-94 for to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest display but how do champions respond? They prevail," Buatsi commented in the arena as he was greeted with boos from the crowd.
The British fighter bounced back after his first professional defeat - an epic battle with his previous opponent in last winter - to claim a 20th career victory.
Various analysts believed the challenger, thirty-one, merited the win but the Derbyshire fighter experienced the second defeat of his 28-bout career.
The Champion Claims Victory in Slow Match
Buatsi's progress has stalled in the past few years through promotional disputes and considerable time of inactivity.
Once tipped as the next star of the British fight scene, he now is positioned trying to achieve expectations - maybe in what seems like a ultimate opportunity - under fresh management.
He started carefully, with both competitors willing to overcommit in a calculated opening few rounds.
Parker, whose single earlier setback came at super middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was competing in only his second appearance at light heavyweight.
By the middle rounds, Parker had stayed in contention, making life awkward for Buatsi, who operated with a solid defense to barely take the better periods.
The audience's tolerance decreased amid frequent holding, with Parker falling to the floor multiple occasions during grappling.
The underdog closed well, but multiple periods were hard to judge. Nevertheless, former world champion the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
The victor progresses, but with the caliber of heavy hitter Benavidez, and Russian star Bivol leading the category, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Homage Inspires Manchester Derby Contest
In the secondary fight, Arthur beat Rea by split-decision in a outstanding local rivalry fight to win the European light heavyweight title.
The arena fell silent for a deeply poignant homage prior to the bout as a bell ceremony sounded in remembrance of local hero Ricky Hatton, who succumbed at age 46 in September.
It was the initial significant broadcast fight night in the urban center since the passing of Hatton. As the last bell faded, the silence became a roar of "only one Hatton".
The fighting during the bout matched the sentiment. Arthur was sent to the canvas by a left hook squarely on the chin in the early round, getting up unsteadily to his position on the referee's count.
But the Manchester United-red attire of the fighter soon pressed ahead again. Across the ring, Rea - in rival City blue - was injured early, noticeable damage forming under his orbital area as the pair traded hard punches.
The challenger, twenty-seven, connected with strong blows but Arthur, thirty-four, matched the intensity with effective uppercuts as the pace rarely dropped over twelve rounds.
The victor narrowly won the result with judges scorecards of even score, close margin and clearer margin, but the two athletes were determined and brave, all the qualities their idol the legend stood for.
