UK light heavyweight Buatsi made a winning yet underwhelming start 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 bout at the boxing arena.
But the challenger had his moments, connecting with powerful clean blows in a fight where the approaches never quite clicked, with excessive holding disrupting the action.
The scoring judges judged it 96-94 to the winner, with the third a 95-95.
"It wasn't my finest showing but how do champions respond? They win," the victor said in the ring as he was met by boos from the spectators.
The British fighter bounced back after his maiden career setback - an memorable fight with Callum Smith in last winter - to earn a twentieth professional win.
Various analysts thought the challenger, 31, deserved the victory but the contender suffered the second loss of his 28-bout career.
Buatsi Claims Triumph in Uneventful Affair
His career development has stalled in recent times through promotional disputes and considerable time of lack of fights.
Earlier forecast as the future of UK boxing, he now stands trying to realize his potential - possibly in what appears to be a ultimate opportunity - under recent support.
He started carefully, with both competitors ready to take risks in a calculated opening few rounds.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in 2022 after damaging his hand in the contest, was competing in only his second fight at light-heavyweight.
By the middle rounds, Parker had kept it close, making life awkward for Buatsi, who worked behind a protective stance to narrowly win the more effective periods.
The spectators' acceptance diminished because of regular grabbing, with Parker going down multiple occasions during grappling.
The underdog ended stronger, but multiple periods were challenging to assess. However, ex-champion Carl Frampton - working with the broadcast network - had it particularly wide for the challenger, awarding Buatsi just only one stanza.
The victor advances, but with the likes of power puncher Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still appears challenging.
Hatton Tribute Inspires Manchester Derby Battle
In the secondary fight, Lyndon Arthur beat Bradley Rea by close verdict in a outstanding Manchester vs Manchester battle to win the continental championship.
The stadium became quiet for a deeply poignant tribute ahead of the contest as ten bells sounded in honor of local hero Hatton, who died aged 46 in last month.
It was the first major televised boxing event in the urban center since Hatton's passing. As the last bell ended, the silence transitioned to a roar of "there's only one Hatton".
The action within the ring matched the emotion. The eventual winner was knocked down by a powerful punch flush on the face in the early round, rising shakily to his feet on the count of eight.
But the traditional red shorts of Arthur soon advanced again. On the opposite side, Rea - in opposing blue - was marked up early, heavy swelling appearing under his eye as the duo landed powerful shots.
The challenger, twenty-seven, landed power punches but Arthur, 34, responded in kind with stinging uppercuts as the intensity hardly diminished over 12 rounds.
The victor narrowly won the decision with official scores of draw, 115-113 and 115-112, but each competitor were relentless and courageous, exactly what their hero the legend embodied.
