UK light-heavyweight Buatsi secured a victory yet underwhelming start to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a split decision in a close ten-round contest at the boxing arena.
But Parker had his moments, delivering effective individual punches in a contest where the techniques failed to mesh, with constant grabbing spoiling the action.
Two judges had it 96-94 for to Buatsi, with the final judge a 95-95.
"It wasn't my best performance but what do champions do? They succeed," Buatsi commented in the arena as he was faced negative reactions from the crowd.
The British fighter bounced back after his first professional defeat - an thrilling contest with Callum Smith in last winter - to claim a twentieth professional win.
Several boxing experts felt the challenger, thirty-one, should have been awarded the decision but the Derbyshire fighter suffered the second loss of his 28-fight professional journey.
Buatsi Secures Victory in Slow Encounter
Buatsi's progress has slowed in recent times through management issues and long spells of inactivity.
Once tipped as the next star of British boxing, he now finds himself trying to realize his potential - possibly in what feels like a last chance - under recent support.
He opened tentatively, with both competitors ready to take risks in a cagey early stages.
Parker, whose only previous defeat came at super middleweight against John Ryder in 2022 after damaging his hand during the bout, was making just his second appearance at the higher weight class.
By the halfway stage, the underdog had made it competitive, making life awkward for Buatsi, who worked behind a solid defense to narrowly win the more effective periods.
The crowd's patience diminished amid constant clinching, with Parker slipping to the canvas several times during close exchanges.
Parker ended stronger, but several stanzas were hard to judge. Although, former world champion Carl Frampton - working with the broadcast network - had it particularly wide for the challenger, awarding Buatsi just only one stanza.
Buatsi advances, but with the likes of power puncher Benavidez, and Russian star Bivol ruling the division, the journey to titles - judging by this showing - still looks steep.
Hatton Tribute Fuels All-Manchester Battle
In the secondary fight, Lyndon Arthur overcame Rea by divided decision in a terrific local rivalry fight to capture the European light-heavyweight championship.
The arena fell silent for a profoundly moving memorial ahead of the contest as ten bells sounded in honor of the city's native Hatton, who succumbed 46 years old in September.
It was the initial significant broadcast boxing event in the location since Hatton's death. As the concluding sound faded, the silence gave way to a roar of "only one Ricky Hatton".
The action within the ring matched the sentiment. Arthur was floored by a significant blow directly to the jaw in the initial stage, rising shakily to his feet on the eight count.
But the traditional red shorts of the fighter soon pressed ahead again. Across the ring, Rea - in rival City blue - was damaged early, heavy swelling appearing under his eye as the duo landed powerful shots.
Rea, 27, delivered effective strikes but Arthur, thirty-four, matched the intensity with stinging uppercuts as the action seldom decreased over twelve rounds.
The victor barely secured the result with judges scorecards of even score, narrow lead and clearer margin, but both fighters were relentless and brave, exactly what their idol the legend embodied.
