British light-heavyweight Buatsi secured a victory yet unimpressive debut to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a judges' verdict in a competitive ten-round fight at the Co-op Live Arena.
But Parker landed significant shots, delivering effective individual punches in a fight where the approaches never quite clicked, with excessive holding marring the action.
Two judges scored it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my best display but how do champions respond? They succeed," Buatsi stated in the ringside as he was faced disapproval from the audience.
The British fighter got back to winning after his initial pro loss - an memorable fight with Callum Smith in last winter - to earn a twentieth professional win.
Various analysts thought Parker, 31, deserved the decision but the contender endured the second loss of his 28-fight professional journey.
Buatsi Secures Victory in Dull Affair
The fighter's advancement has stalled in recent times through contract problems and long spells of inactivity.
Earlier forecast as the next star of British boxing, he now is positioned trying to achieve expectations - possibly in what feels like a ultimate opportunity - under fresh management.
He opened tentatively, with neither fighter ready to be aggressive in a calculated initial rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in two years ago after damaging his hand in the contest, was participating in his second appearance at the higher weight class.
By the halfway stage, the underdog had made it competitive, creating difficulties for Buatsi, who operated with a solid defense to edge the more effective periods.
The audience's tolerance diminished because of frequent holding, with Parker going down various instances during grappling.
The underdog finished the stronger, but multiple periods were hard to judge. Although, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for Parker, giving Buatsi just only one stanza.
Buatsi advances, but with the likes of hard hitter Benavidez, and Russian champion Dmitry Bivol ruling the division, the path to championship success - on this evidence - still appears challenging.
Ricky Hatton Memorial Inspires Local Rivalry Fight
In the co-main event, Lyndon Arthur beat Rea by divided decision in a outstanding all-Manchester contest to secure the continental title.
The venue became quiet for a deeply poignant memorial before the fight as a bell ceremony rang in honor of the city's native Ricky Hatton, who succumbed at age 46 in September.
It was the initial significant broadcast fight night in the city since the passing of Hatton. As the final chime disappeared, the silence gave way to a roar of "there's just one Ricky Hatton".
The fighting within the ring reflected the sentiment. Arthur was floored by a powerful punch squarely on the face in the second round, rising shakily to his feet on the referee's count.
But the traditional red shorts of the fighter soon advanced again. From the other corner, Rea - in competitive blue - was injured quickly, heavy swelling appearing under his eye as the duo landed powerful shots.
The challenger, 27, landed power punches but Arthur, thirty-four, fought fire with fire with effective uppercuts as the pace rarely dropped over 12 rounds.
The victor edged the verdict with scoring tallies of draw, narrow lead and wider gap, but both fighters were determined and full of heart, everything their hero the legend represented.
