UK light heavyweight Buatsi secured a victory yet underwhelming start to life under new promoters with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a judges' verdict in a competitive ten-round contest at the Co-op Live Arena.
But the challenger had his moments, landing some telling single shots in a contest where the styles never quite clicked, with frequent clinching spoiling the contest.
Two judges had it 96-94 to the winner, with the third a 95-95 draw.
"It wasn't my finest display but what do champions do? They win," Buatsi commented in the ring as he was met by negative reactions from the crowd.
The Londoner bounced back after his maiden career setback - an epic battle with Callum Smith in February - to earn a twentieth professional win.
Various analysts felt the challenger, 31, deserved the decision but the British boxer endured the second loss of his 28-bout career.
Buatsi Earns Win in Slow Encounter
The fighter's advancement has slowed in recent times through contract problems and long spells of lack of fights.
Earlier forecast as the rising talent of UK boxing, he now is positioned trying to realize his potential - perhaps in what appears to be a last chance - under new promotional backing.
He opened tentatively, with neither fighter ready to be aggressive in a tentative opening few rounds.
Parker, whose only previous defeat came at super-middleweight against his former opponent in two years ago after damaging his hand in the contest, was making just his second fight at the higher weight class.
By the halfway stage, the underdog had made it competitive, creating difficulties for the favorite, who worked behind a solid defense to barely take the better periods.
The audience's tolerance diminished amid frequent holding, with Parker going down several times during clinches.
The underdog finished the stronger, but multiple periods were hard to judge. Nevertheless, former world champion the boxing analyst - working with Dazn - had it strongly leaning for the challenger, scoring for Buatsi just only one stanza.
The victor progresses, but with the caliber of power puncher Benavidez, and Russian star Dmitry Bivol dominating the weight class, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Homage Fuels Manchester Derby Contest
In the secondary fight, Lyndon Arthur beat Rea by split-decision in a terrific Manchester vs Manchester contest to capture the European light-heavyweight title.
The stadium grew hushed for a deeply poignant tribute before the fight as 10 bells sounded in remembrance of local hero Ricky Hatton, who succumbed aged 46 in September.
It was the premier televised boxing event in the location since Hatton's passing. As the final chime ended, the silence became a chant of "there's just one Hatton".
The fighting during the bout reflected the sentiment. The eventual winner was sent to the canvas by a significant blow flush on the chin in the early round, standing uncertainly to his footing on the count of eight.
But the traditional red equipment of the fighter soon came forward again. On the opposite side, Rea - in opposing blue - was marked up quickly, significant bruising forming under his eye as the pair landed hard punches.
The challenger, 27, landed power punches but the champion, 34, responded in kind with stinging uppercuts as the intensity seldom decreased over 12 rounds.
The victor narrowly won the decision with scoring tallies of even score, narrow lead and 115-112, but the two athletes were relentless and brave, exactly what their idol Hatton represented.
