UK light heavyweight Buatsi made a winning yet disappointing start to life under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a tight ten-round contest at the boxing arena.
But Parker landed significant shots, connecting with powerful single shots in a fight where the styles didn't complement each other, with excessive holding spoiling the action.
Two judges had it 96-94 in favor to Buatsi, with the third a 95-95.
"It wasn't my best performance but what do champions do? They succeed," the victor stated in the arena as he was met by disapproval from the audience.
The Londoner bounced back after his first professional defeat - an memorable fight with his previous opponent in last winter - to secure a twentieth professional win.
Various analysts believed the challenger, thirty-one, should have been awarded the victory but the British boxer endured the second defeat of his 28-bout career.
The Champion Earns Triumph in Uneventful Encounter
The fighter's advancement has stalled in recent times through promotional disputes and long spells of lack of fights.
Once tipped 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 opened tentatively, with both competitors prepared to be aggressive in a tentative opening few rounds.
Parker, whose only previous defeat came at super-middleweight against John Ryder in two years ago after damaging his hand mid-fight, was competing in only his second fight at the higher weight class.
By the fight's midpoint, Parker had made it competitive, making life awkward for Buatsi, who operated with a protective stance to edge the better rounds.
The audience's tolerance decreased during regular grabbing, with the challenger falling to the floor various instances during clinches.
The underdog finished the stronger, but several stanzas were difficult to score. However, previous titleholder Carl Frampton - working with the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just a single round.
Buatsi moves on, but with the presence of power puncher Benavidez, and Russian star Dmitry Bivol leading the category, the path to championship success - on this evidence - still appears challenging.
Ricky Hatton Homage Fuels Manchester Derby Contest
In the supporting bout, Arthur defeated Bradley Rea by close verdict in a excellent local rivalry fight to secure the European light-heavyweight championship.
The venue fell silent for a profoundly moving homage ahead of the contest as a bell ceremony sounded in remembrance of Manchester's own Ricky Hatton, who passed away 46 years old in recently.
It was the first major televised match card in the city since Hatton's death. As the last bell faded, the stillness gave way to a chant of "there's only one Hatton".
The competition inside the ropes equaled the sentiment. Arthur was sent to the canvas by a left hook squarely on the chin in the early round, rising shakily to his footing on the referee's count.
But the traditional red shorts of the fighter soon came forward again. Across the ring, the opponent - in competitive blue - was injured soon, heavy swelling developing under his eye as the fighters exchanged punishing blows.
Rea, 27, delivered effective strikes but the champion, thirty-four, matched the intensity with powerful upward punches as the intensity seldom decreased over twelve rounds.
Arthur narrowly won the decision with judges scorecards of even score, close margin and clearer margin, but the two athletes were persistent and brave, everything their inspiration Hatton stood for.
