UK light heavyweight Buatsi earned a win yet disappointing start to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output seemingly secured him a judges' verdict in a competitive ten-round bout at the Co-op Live Arena.
But the challenger had his moments, delivering effective individual punches in a contest where the styles failed to mesh, with constant grabbing disrupting the contest.
Two judges scored it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my greatest display but what defines champions? They prevail," the victor commented in the ring as he was greeted with negative reactions from the audience.
The British fighter returned to winning ways after his first professional defeat - an memorable fight with his previous opponent in February - to earn a twentieth professional win.
Various analysts thought the challenger, 31, should have been awarded the victory but the Derbyshire fighter experienced the second defeat of his 28-bout career.
The Champion Secures Triumph in Slow Encounter
The fighter's advancement has slowed in recent years through management issues and long spells of lack of fights.
Earlier forecast as the rising talent of the British fight scene, he now stands trying to achieve expectations - possibly in what seems like a final roll of the dice - under new promotional backing.
He started carefully, with both competitors ready to take risks in a calculated opening few rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was competing in only his second bout at light-heavyweight.
By the middle rounds, Parker had kept it close, causing problems for the favorite, who utilized a protective stance to narrowly win the more effective periods.
The spectators' acceptance decreased amid constant clinching, with the challenger falling to the floor several times during close exchanges.
The underdog closed well, but multiple periods were difficult to score. Although, former world champion Carl Frampton - commentating on the broadcast network - had it strongly leaning for Parker, awarding Buatsi just one round.
The victor moves on, but with the presence of power puncher Benavidez, and Eastern European fighter Bivol leading the category, the path to championship success - judging by this showing - still seems difficult.
Ricky Hatton Tribute Fuels All-Manchester Contest
In the co-main event, Arthur overcame Rea by split-decision in a excellent all-Manchester contest to secure the European light-heavyweight belt.
The venue became quiet for a profoundly moving tribute prior to the bout as ten bells sounded in memory of local hero Ricky Hatton, who died aged 46 in recently.
It was the first major televised match card in the city since Hatton's passing. As the final chime ended, the silence became a roar of "only one Hatton".
The fighting inside the ropes reflected the sentiment. The eventual winner was knocked down by a left hook directly to the chin in the second round, standing uncertainly to his feet on the count of eight.
But the red-colored shorts of Arthur soon advanced again. Across the ring, the opponent - in opposing blue - was injured early, heavy swelling developing under his eye as the duo exchanged powerful shots.
The challenger, twenty-seven, delivered effective strikes but the champion, 34, responded in kind with powerful upward punches as the action rarely dropped over twelve rounds.
The victor edged the decision with judges scorecards of even score, narrow lead and clearer margin, but the two athletes were persistent and full of heart, exactly what their idol Hatton stood for.
