British light heavyweight Buatsi earned a win yet unimpressive debut to life 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 judges' verdict in a close 10-round contest at the Co-op Live Arena.
But the challenger found success, delivering effective individual punches in a fight where the styles failed to mesh, with constant grabbing spoiling the contest.
The scoring judges judged it 96-94 for to the winner, with the remaining official a 95-95.
"It wasn't my best display but how do champions respond? They succeed," the victor stated in the ringside as he was met by negative reactions from the spectators.
The British fighter got back to winning after his first professional defeat - an epic battle with Callum Smith in February - to earn a 20th pro success.
Various analysts thought Parker, thirty-one, should have been awarded the victory but the Derbyshire fighter endured the second setback of his 28-fight career.
The Champion Earns Victory in Slow Affair
Buatsi's progress has slowed in recent years through management issues and long spells of lack of fights.
Earlier forecast as the future of the British fight scene, he now finds himself trying to achieve expectations - perhaps in what appears to be a final roll of the dice - under new promotional backing.
He began cautiously, with both competitors prepared to take risks in a tentative opening few rounds.
The challenger, whose sole prior loss came at super middleweight against his former opponent in two years ago after breaking his hand mid-fight, was participating in his second bout at light-heavyweight.
By the fight's midpoint, Parker had kept it close, creating difficulties for Buatsi, who operated with a protective stance to edge the more effective rounds.
The spectators' acceptance wore thin because of constant clinching, with the challenger falling to the floor several times during close exchanges.
The underdog finished the stronger, but several stanzas were difficult to score. Nevertheless, previous titleholder the boxing analyst - working with the broadcast network - had it significantly in favor for Parker, giving Buatsi just only one stanza.
The victor advances, but with the likes of power puncher Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Homage Motivates Manchester Derby Battle
In the supporting bout, Arthur defeated Rea by close verdict in a excellent Manchester vs Manchester battle to capture the continental championship.
The arena became quiet for a deeply poignant tribute ahead of the contest as a bell ceremony rang in honor of the city's native Ricky Hatton, who succumbed at age 46 in last month.
It was the initial significant broadcast boxing event in the urban center since Hatton's death. As the final chime ended, the silence transitioned to a roar of "only one Hatton".
The competition during the bout reflected the sentiment. The eventual winner was knocked down by a left hook directly to the chin in the early round, rising shakily to his footing on the referee's count.
But the red-colored shorts of Arthur soon came forward again. Across the ring, Rea - in rival City blue - was damaged quickly, heavy swelling appearing under his face as the fighters exchanged powerful shots.
Rea, twenty-seven, landed power punches but Arthur, 34, fought fire with fire with powerful upward punches as the action hardly diminished over the full distance.
The victor edged the decision with judges scorecards of draw, close margin and wider gap, but both fighters were persistent and courageous, exactly what their hero Hatton stood for.
