British light-heavyweight Buatsi made a winning yet disappointing beginning to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a majority decision in a competitive 10-round fight at the Manchester venue.
But the challenger landed significant shots, connecting with powerful clean blows in a contest where the techniques failed to mesh, with constant grabbing spoiling the action.
The officials judged it 96-94 to Buatsi, with the final judge a 95-95.
"It wasn't my best showing but how do champions respond? They win," Buatsi stated in the ring as he was met by disapproval from the audience.
The British fighter bounced back after his first professional defeat - an epic battle with Callum Smith in February - to claim a twentieth professional win.
Various analysts felt the challenger, 31, merited the victory but the British boxer suffered the second defeat of his 28-fight career.
The Champion Secures Triumph in Slow Encounter
Buatsi's progress has stalled in recent years through contract problems and considerable time of lack of fights.
Once tipped as the rising talent of British boxing, he now is positioned trying to achieve expectations - possibly in what appears to be a last chance - under new promotional backing.
He started carefully, with the two boxers prepared to be aggressive in a tentative early stages.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after breaking his hand mid-fight, was competing in only his second appearance at light-heavyweight.
By the halfway stage, Parker had kept it close, causing problems for the favorite, who operated with a protective stance to edge the cleaner rounds.
The crowd's patience decreased during frequent holding, with the challenger falling to the floor multiple occasions during close exchanges.
The underdog closed well, but multiple periods were hard to judge. However, former world champion the boxing analyst - working with Dazn - had it particularly wide for the challenger, scoring for Buatsi just a single round.
The victor progresses, but with the likes of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the road to world glory - based on this performance - still looks steep.
Ricky Hatton Memorial Inspires Manchester Derby Battle
In the supporting bout, Lyndon Arthur defeated Bradley Rea by close verdict in a outstanding local rivalry battle to win the continental championship.
The venue fell silent for a profoundly moving tribute prior to the bout as ten bells sounded in memory of Manchester's own Hatton, who died aged 46 in September.
It was the first major televised fight night in the city since Hatton's passing. As the concluding sound disappeared, the quiet became a shout of "there's just one Hatton".
The competition inside the ropes equaled the emotion. The eventual winner was floored by a left hook directly to the face in the initial stage, rising shakily to his feet on the count of eight.
But the red-colored attire of the fighter soon pressed ahead again. On the opposite side, Rea - in rival City blue - was marked up soon, heavy swelling developing under his orbital area as the fighters exchanged punishing blows.
The challenger, 27, landed power punches but Arthur, thirty-four, fought fire with fire with effective uppercuts as the pace hardly diminished over 12 rounds.
Arthur edged the result with official scores of draw, 115-113 and 115-112, but the two athletes were persistent and full of heart, everything their idol the legend stood for.
