UK light-heavyweight Buatsi made a winning yet underwhelming start to his career under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's activity level seemingly secured him a judges' verdict in a tight ten-round bout at the Co-op Live Arena.
But the challenger found success, landing some telling clean blows in a match where the techniques failed to mesh, with excessive holding disrupting the fight.
The officials judged it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my greatest display but how do champions respond? They win," Buatsi stated in the arena as he was faced negative reactions from the crowd.
The Londoner bounced back after his initial pro loss - an memorable fight with Callum Smith in February - to claim a twentieth professional win.
Several boxing experts thought Parker, 31, deserved the decision but the British boxer experienced the second defeat of his 28-bout professional journey.
Buatsi Claims Win in Uneventful Match
The fighter's advancement has plateaued in the past few years through contract problems and long spells of inactivity.
Once tipped as the rising talent of British boxing, he now stands trying to fulfil that early promise - possibly in what feels like a last chance - under recent support.
He began cautiously, with neither fighter prepared to take risks in a cagey early stages.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after suffering a hand injury in the contest, was competing in only his second appearance at the higher weight class.
By the middle rounds, Parker had kept it close, causing problems for Buatsi, who operated with a tight guard to narrowly win the better periods.
The spectators' acceptance wore thin amid regular grabbing, with Parker falling to the floor various instances during clinches.
The underdog ended stronger, but several stanzas were challenging to assess. Nevertheless, previous titleholder the boxing analyst - commentating on Dazn - had it particularly wide for the challenger, giving Buatsi just only one stanza.
The victor moves on, but with the likes of hard hitter Benavidez, and Russian champion Bivol leading the category, the path to championship success - on this evidence - still appears challenging.
Ricky Hatton Homage Inspires Local Rivalry Battle
In the secondary fight, Lyndon Arthur overcame Rea by divided decision in a outstanding all-Manchester contest to win the continental championship.
The venue fell silent for a deeply poignant memorial prior to the bout as ten bells sounded in remembrance of local hero Hatton, who passed away at age 46 in September.
It was the premier televised boxing event in the city since Hatton's passing. As the final chime ended, the stillness became a chant of "there's just one Ricky Hatton".
The competition during the bout equaled the emotion. The eventual winner was knocked down by a left hook flush on the chin in the initial stage, getting up unsteadily to his position on the eight count.
But the red-colored attire of the fighter soon advanced again. On the opposite side, Rea - in opposing blue - was damaged early, significant bruising appearing under his face as the fighters traded punishing blows.
Rea, 27, connected with strong blows but the champion, thirty-four, fought fire with fire with effective uppercuts as the action rarely dropped over the full distance.
The victor barely secured the decision with scoring tallies of 114-114, 115-113 and clearer margin, but both fighters were persistent and full of heart, exactly what their inspiration Hatton embodied.
