UK light-heavyweight Buatsi earned a win yet disappointing start to his career under recent management with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate seemingly secured him a judges' verdict in a tight 10-round fight at the boxing arena.
But the challenger found success, connecting with powerful single shots in a fight where the techniques never quite clicked, with constant grabbing disrupting the contest.
The scoring judges judged it 96-94 in favor to Buatsi, with the final judge a 95-95.
"It wasn't my best performance but what defines champions? They win," Buatsi said in the ringside as he was greeted with negative reactions from the spectators.
The Londoner got back to winning after his maiden career setback - an epic battle with his previous opponent in February - to claim a twentieth professional win.
Several boxing experts thought the challenger, thirty-one, should have been awarded the decision but the British boxer endured the second loss of his 28-bout professional journey.
The Champion Claims Win in Uneventful Encounter
Buatsi's progress has slowed in recent years through promotional disputes and long spells of inactivity.
Previously predicted as the next star of the British fight scene, he now finds himself trying to realize his potential - maybe in what feels like a last chance - under new promotional backing.
He started carefully, with the two boxers prepared to take risks in a tentative initial rounds.
Parker, whose only previous defeat came at super middleweight against John Ryder in 2022 after damaging his hand during the bout, was participating in his second bout at light heavyweight.
By the middle rounds, Parker had kept it close, making life awkward for the favorite, who worked behind a protective stance to narrowly win the cleaner rounds.
The audience's tolerance decreased during constant clinching, with Parker falling to the floor multiple occasions during clinches.
The underdog ended stronger, but many rounds were challenging to assess. Although, ex-champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just one round.
The victor moves on, but with the caliber of power puncher David Benavidez, and Eastern European fighter Bivol leading the category, the path to championship success - judging by this showing - still looks steep.
Ricky Hatton Memorial Fuels Manchester Derby Fight
In the supporting bout, Lyndon Arthur overcame Bradley Rea by split-decision in a terrific all-Manchester contest to capture the European light heavyweight title.
The stadium grew hushed for a profoundly moving tribute prior to the bout as 10 bells tolled in remembrance of Manchester's own Ricky Hatton, who passed away aged 46 in last month.
It was the initial significant broadcast fight night in the city since Hatton's passing. As the last bell faded, the silence transitioned to a shout of "there's just one Ricky Hatton".
The action during the bout matched the sentiment. The eventual winner was floored by a significant blow squarely on the face in the early round, standing uncertainly to his feet on the count of eight.
But the traditional red attire of Arthur soon pressed ahead again. From the other corner, the opponent - in rival City blue - was injured early, noticeable damage appearing under his eye as the duo landed powerful shots.
The challenger, 27, landed power punches but the champion, thirty-four, matched the intensity with powerful upward punches as the action seldom decreased over 12 rounds.
Arthur barely secured the result with judges scorecards of draw, 115-113 and clearer margin, but each competitor were determined and full of heart, exactly what their inspiration Hatton embodied.
