UK light-heavyweight Buatsi secured a victory yet underwhelming debut to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a close 10-round fight at the Co-op Live Arena.
But Parker had his moments, landing some telling single shots in a contest where the techniques failed to mesh, with frequent clinching disrupting the fight.
The officials scored it 96-94 in favor to the winner, with the third a even score.
"It wasn't my finest performance but how do champions respond? They win," Buatsi commented in the ring as he was met by negative reactions from the audience.
The British fighter bounced back after his maiden career setback - an epic battle with Callum Smith in last winter - to earn a 20th pro success.
Some ringside observers believed Parker, thirty-one, should have been awarded the victory but the British boxer experienced the second defeat of his 28-match career.
The Champion Earns Win in Slow Affair
His career development has slowed in the past few years through promotional disputes and extended periods of inactivity.
Once tipped as the rising talent of the British fight scene, he now is positioned trying to fulfil that early promise - possibly in what seems like a last chance - under fresh management.
He began cautiously, with neither fighter willing to overcommit in a calculated opening few rounds.
Parker, whose single earlier setback came at super middleweight against his former opponent in two years ago after suffering a hand injury mid-fight, was competing in only his second appearance at light-heavyweight.
By the middle rounds, Parker had stayed in contention, creating difficulties for the favorite, who worked behind a solid defense to edge the more effective periods.
The spectators' acceptance decreased during regular grabbing, with Parker falling to the floor various instances during clinches.
The underdog finished the stronger, but several stanzas were challenging to assess. Although, ex-champion the boxing analyst - working with Dazn - had it significantly in favor for the challenger, awarding Buatsi just only one stanza.
The victor moves on, but with the presence of power puncher David Benavidez, and Eastern European fighter Bivol dominating the weight class, the road to world glory - judging by this showing - still seems difficult.
Hatton Homage Motivates All-Manchester Fight
In the supporting bout, Arthur beat Bradley Rea by divided decision in a outstanding Manchester vs Manchester battle to capture the European light-heavyweight championship.
The arena fell silent for a deeply poignant tribute before the fight as ten bells rang in remembrance of Manchester's own Ricky Hatton, who succumbed at age 46 in last month.
It was the first major televised match card in the urban center since the passing of Hatton. As the concluding sound disappeared, the quiet transitioned to a roar of "there's just one Ricky Hatton".
The fighting inside the ropes matched the sentiment. The eventual winner was floored by a left hook squarely on the face in the second round, rising shakily to his position on the count of eight.
But the Manchester United-red attire of Arthur soon came forward again. On the opposite side, Rea - in rival City blue - was damaged quickly, significant bruising forming under his face as the duo landed powerful shots.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with powerful upward punches as the action rarely dropped over the full distance.
Arthur edged the result with official scores of even score, narrow lead and clearer margin, but the two athletes were relentless and courageous, all the qualities their hero the legend stood for.
