UK light heavyweight Buatsi made a winning yet underwhelming start to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a tight ten-round contest at the boxing arena.
But Parker landed significant shots, connecting with powerful clean blows in a fight where the styles never quite clicked, with frequent clinching marring the fight.
The officials scored it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my best performance but how do champions respond? They prevail," the victor commented in the arena as he was faced boos from the crowd.
The Londoner returned to winning ways after his initial pro loss - an memorable fight with his previous opponent in last winter - to earn a 20th pro success.
Some ringside observers thought Parker, thirty-one, should have been awarded the victory but the Derbyshire fighter experienced the second setback of his 28-bout professional journey.
Buatsi Earns Win in Uneventful Encounter
His career development has stalled in recent years through promotional disputes and long spells of lack of fights.
Earlier forecast as the next star of British boxing, he now stands trying to achieve expectations - perhaps in what appears to be a ultimate opportunity - under new promotional backing.
He started carefully, with both competitors prepared to overcommit in a cagey initial rounds.
Parker, whose only previous defeat came at super-middleweight against his former opponent in two years ago after damaging his hand mid-fight, was competing in only his second appearance at light heavyweight.
By the middle rounds, the underdog had stayed in contention, causing problems for Buatsi, who worked behind a tight guard to edge the better stanzas.
The audience's tolerance wore thin amid regular grabbing, with the challenger slipping to the canvas several times during grappling.
Parker closed well, but multiple periods were difficult to score. Nevertheless, ex-champion the boxing analyst - providing analysis for Dazn - had it significantly in favor for Parker, scoring for Buatsi just a single round.
The victor progresses, but with the presence of heavy hitter Benavidez, and Russian champion Bivol dominating the weight class, the journey to titles - based on this performance - still appears challenging.
Ricky Hatton Homage Fuels Manchester Derby Contest
In the supporting bout, Arthur defeated Bradley Rea by close verdict in a outstanding local rivalry fight to secure the continental belt.
The arena fell silent for a profoundly moving memorial prior to the bout as a bell ceremony rang in remembrance of Manchester's own Hatton, who succumbed 46 years old in last month.
It was the first major televised match card in the location since Hatton's death. As the final chime ended, the quiet gave way to a shout of "there's just one Ricky Hatton".
The fighting within the ring reflected the sentiment. The eventual winner was sent to the canvas by a left hook directly to the face in the initial stage, rising shakily to his footing on the count of eight.
But the traditional red equipment of Arthur soon pressed ahead again. Across the ring, Rea - in competitive blue - was damaged quickly, heavy swelling forming under his orbital area as the fighters exchanged punishing blows.
The challenger, twenty-seven, connected with strong blows but Arthur, thirty-four, matched the intensity with stinging uppercuts as the intensity hardly diminished over 12 rounds.
Arthur edged the result with judges scorecards of even score, narrow lead and clearer margin, but each competitor were relentless and courageous, all the qualities their inspiration Hatton embodied.
