UK light heavyweight Buatsi made a winning yet disappointing debut to his career under recent management with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a tight ten-round bout at the Manchester venue.
But the challenger landed significant shots, delivering effective clean blows in a contest where the styles failed to mesh, with frequent clinching disrupting the fight.
Two judges judged it 96-94 in favor to Buatsi, with the final judge a even score.
"It wasn't my finest showing but how do champions respond? They win," the victor commented in the ring as he was faced disapproval from the audience.
The Londoner bounced back after his first professional defeat - an thrilling contest with Callum Smith in last winter - to earn a 20th pro success.
Several boxing experts believed the challenger, thirty-one, deserved the decision but the British boxer suffered the second loss of his 28-bout professional journey.
The Champion Secures Win in Uneventful Encounter
His career development has stalled in recent years through promotional disputes and considerable time of inactivity.
Earlier forecast as the next star of British boxing, he now stands trying to realize his potential - perhaps in what feels like a final roll of the dice - under fresh management.
He began cautiously, with neither fighter willing to take risks in a cagey opening few rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after suffering a hand injury in the contest, was making just his second appearance at light heavyweight.
By the fight's midpoint, the underdog had made it competitive, making life awkward for the favorite, who utilized a solid defense to edge the more effective rounds.
The audience's tolerance wore thin because of constant clinching, with the challenger slipping to the canvas various instances during close exchanges.
The underdog ended stronger, but many rounds were challenging to assess. However, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for the challenger, giving Buatsi just a single round.
The victor moves on, but with the presence of power puncher David Benavidez, and Russian champion Bivol ruling the division, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Homage Inspires Manchester Derby Contest
In the secondary fight, Lyndon Arthur overcame Rea by divided decision in a terrific Manchester vs Manchester contest to capture the continental championship.
The venue became quiet for a deeply poignant tribute ahead of the contest as 10 bells tolled in honor of Manchester's own Ricky Hatton, who died 46 years old in recently.
It was the first major televised fight night in the location since Hatton's passing. As the concluding sound disappeared, the stillness transitioned to a roar of "there's just one Ricky Hatton".
The fighting within the ring equaled the feeling. The eventual winner was knocked down by a powerful punch squarely on the jaw in the initial stage, standing uncertainly to his feet on the referee's count.
But the traditional red shorts of Arthur soon pressed ahead again. From the other corner, the opponent - in opposing blue - was marked up soon, heavy swelling developing under his orbital area as the duo landed punishing blows.
Rea, 27, landed power punches but the champion, 34, matched the intensity with stinging uppercuts as the intensity seldom decreased over the full distance.
Arthur edged the result with scoring tallies of even score, 115-113 and wider gap, but both fighters were persistent and courageous, everything their idol the legend stood for.
