UK light-heavyweight Buatsi secured a victory yet unimpressive debut to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a split decision in a tight 10-round contest at the Manchester venue.
But Parker found success, delivering effective individual punches in a contest where the styles didn't complement each other, with constant grabbing marring the contest.
The officials judged it 96-94 in favor to the winner, with the remaining official a 95-95 draw.
"It wasn't my best display but how do champions respond? They win," the victor commented in the ring as he was met by negative reactions from the spectators.
The Londoner returned to winning ways after his first professional defeat - an thrilling contest with his previous opponent in last winter - to earn a twentieth professional win.
Several boxing experts felt the challenger, 31, should have been awarded the victory but the British boxer suffered the second loss of his 28-match professional journey.
The Champion Earns Triumph in Dull Encounter
Buatsi's progress has stalled in recent years through contract problems and extended periods of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now stands trying to achieve expectations - maybe in what feels like a last chance - under recent support.
He opened tentatively, with the two boxers prepared to take risks in a tentative opening few rounds.
Parker, whose single earlier setback came at the lower weight class against his former opponent in 2022 after suffering a hand injury mid-fight, was competing in only his second bout at light heavyweight.
By the fight's midpoint, Parker had kept it close, creating difficulties for the favorite, who operated with a tight guard to narrowly win the better periods.
The audience's tolerance diminished amid frequent holding, with the challenger going down multiple occasions during close exchanges.
The underdog finished the stronger, but many rounds were difficult to score. Although, former world champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for the challenger, giving Buatsi just one round.
The victor advances, but with the presence of power puncher Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - based on this performance - still seems difficult.
Ricky Hatton Memorial Motivates All-Manchester Contest
In the supporting bout, Lyndon Arthur beat Rea by split-decision in a terrific all-Manchester battle to secure the continental belt.
The arena became quiet for a deeply poignant tribute ahead of the contest as a bell ceremony sounded in honor of Manchester's own Ricky Hatton, who passed away at age 46 in last month.
It was the first major televised fight night in the urban center since Hatton's death. As the concluding sound disappeared, the quiet gave way to a shout of "there's just one Hatton".
The action within the ring matched the feeling. Arthur was sent to the canvas by a powerful punch squarely on the jaw in the early round, rising shakily to his footing on the referee's count.
But the traditional red equipment of Arthur soon came forward again. On the opposite side, Rea - in opposing blue - was damaged early, noticeable damage developing under his orbital area as the pair exchanged hard punches.
The challenger, 27, landed power punches but the champion, 34, fought fire with fire with powerful upward punches as the intensity seldom decreased over the full distance.
Arthur edged the verdict with official scores of 114-114, 115-113 and 115-112, but the two athletes were relentless and full of heart, everything their idol the legend embodied.
