UK light heavyweight Buatsi made a winning yet unimpressive debut to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a judges' verdict in a close ten-round fight at the boxing arena.
But Parker landed significant shots, connecting with powerful clean blows in a contest where the styles never quite clicked, with excessive holding spoiling the contest.
The scoring judges had it 96-94 for to Buatsi, with the final judge a even score.
"It wasn't my finest showing but how do champions respond? They win," the victor stated in the ringside as he was faced negative reactions from the crowd.
The British fighter returned to winning ways after his maiden career setback - an thrilling contest with Callum Smith in last winter - to claim a twentieth professional win.
Some ringside observers believed Parker, 31, should have been awarded the victory but the British boxer experienced the second loss of his 28-match career.
Buatsi Secures Win in Uneventful Match
His career development has plateaued in recent years through contract problems and extended periods of inactivity.
Previously predicted as the next star of UK boxing, he now is positioned trying to fulfil that early promise - maybe in what feels like a last chance - under recent support.
He opened tentatively, with neither fighter willing to be aggressive in a cagey initial rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in two years ago after suffering a hand injury during the bout, was participating in his second appearance at the higher weight class.
By the halfway stage, Parker had stayed in contention, making life awkward for Buatsi, who utilized a solid defense to barely take the better rounds.
The spectators' acceptance diminished because of constant clinching, with the challenger going down several times during close exchanges.
The underdog finished the stronger, but multiple periods were hard to judge. Although, former world champion the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just one round.
The victor advances, but with the likes of power puncher David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - based on this performance - still seems difficult.
Ricky Hatton Homage Fuels All-Manchester Fight
In the co-main event, Lyndon Arthur beat Rea by divided decision in a outstanding local rivalry contest to secure the European light heavyweight belt.
The arena grew hushed for a deeply poignant tribute ahead of the contest as a bell ceremony tolled in honor of the city's native Hatton, who passed away 46 years old in last month.
It was the first major televised fight night in the city since the passing of Hatton. As the last bell ended, the silence gave way to a chant of "only one Hatton".
The fighting inside the ropes reflected the emotion. Arthur was sent to the canvas by a left hook directly to the face in the early round, standing uncertainly to his feet on the count of eight.
But the Manchester United-red equipment of Arthur soon pressed ahead again. From the other corner, the opponent - in competitive blue - was marked up soon, noticeable damage appearing under his eye as the fighters traded powerful shots.
Rea, twenty-seven, landed power punches but the champion, thirty-four, fought fire with fire with powerful upward punches as the action seldom decreased over the full distance.
The victor narrowly won the result with scoring tallies of 114-114, narrow lead and clearer margin, but both fighters were persistent and brave, everything their inspiration the legend embodied.
