British light-heavyweight Buatsi made a winning yet unimpressive start to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output seemingly secured him a judges' verdict in a competitive ten-round fight at the boxing arena.
But the challenger had his moments, connecting with powerful individual punches in a contest where the approaches failed to mesh, with constant grabbing disrupting the action.
The officials judged it 96-94 in favor to the winner, with the final judge a 95-95.
"It wasn't my greatest showing but how do champions respond? They win," the victor said in the arena as he was met by negative reactions from the spectators.
The British fighter bounced back after his maiden career setback - an epic battle with his previous opponent in last winter - to earn a twentieth professional win.
Various analysts believed the challenger, thirty-one, deserved the victory but the contender suffered the second setback of his 28-match professional journey.
Buatsi Earns Victory in Uneventful Affair
Buatsi's progress has slowed in recent times through contract problems and long spells of lack of fights.
Earlier forecast as the rising talent of the British fight scene, he now stands trying to realize his potential - possibly in what feels like a ultimate opportunity - under recent support.
He started carefully, with both competitors prepared to take risks in a calculated initial rounds.
Parker, whose sole prior loss came at super middleweight against John Ryder in 2022 after breaking his hand mid-fight, was making just his second bout at light-heavyweight.
By the fight's midpoint, Parker had made it competitive, causing problems for the favorite, who worked behind a protective stance to edge the more effective stanzas.
The spectators' acceptance wore thin because of regular grabbing, with Parker slipping to the canvas various instances during grappling.
Parker finished the stronger, but many rounds were challenging to assess. Although, ex-champion Carl Frampton - providing analysis for Dazn - had it strongly leaning for the challenger, giving Buatsi just one round.
Buatsi advances, but with the caliber of hard hitter Benavidez, and Russian champion Dmitry Bivol ruling the division, the path to championship success - on this evidence - still looks steep.
Ricky Hatton Homage Fuels All-Manchester Contest
In the secondary fight, Arthur defeated Rea by close verdict in a outstanding Manchester vs Manchester contest to secure the European light-heavyweight title.
The arena fell silent for a extremely emotional memorial before the fight as ten bells sounded in honor of local hero Ricky Hatton, who passed away aged 46 in recently.
It was the premier televised match card in the city since the passing of Hatton. As the concluding sound ended, the stillness gave way to a chant of "there's only one Hatton".
The competition inside the ropes equaled the feeling. Arthur was sent to the canvas by a left hook squarely on the chin in the early round, rising shakily to his feet on the count of eight.
But the red-colored attire of the fighter soon pressed ahead again. From the other corner, the opponent - in opposing blue - was damaged soon, noticeable damage forming under his orbital area as the duo landed powerful shots.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, fought fire with fire with powerful upward punches as the pace hardly diminished over 12 rounds.
Arthur narrowly won the verdict with judges scorecards of 114-114, narrow lead and 115-112, but both fighters were determined and brave, everything their idol Hatton embodied.
