British light heavyweight Buatsi made a winning yet unimpressive debut to life under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a majority decision in a tight ten-round contest at the Manchester venue.
But Parker landed significant shots, delivering effective single shots in a contest where the techniques failed to mesh, with constant grabbing spoiling the action.
Two judges scored it 96-94 in favor to Buatsi, with the third a 95-95 draw.
"It wasn't my best display but what do champions do? They succeed," the victor said in the ringside as he was met by boos from the spectators.
The British fighter got back to winning after his first professional defeat - an memorable fight with his previous opponent in last winter - to earn a twentieth professional win.
Several boxing experts thought Parker, thirty-one, merited the decision but the contender endured the second defeat of his 28-match professional journey.
The Champion Earns Victory in Dull Match
Buatsi's progress has plateaued in recent times through promotional disputes and long spells of lack of fights.
Previously predicted as the next star of British boxing, he now is positioned trying to achieve expectations - perhaps in what seems like a last chance - under new promotional backing.
He opened tentatively, with the two boxers ready to be aggressive in a tentative early stages.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in 2022 after breaking his hand in the contest, was participating in his second fight at light-heavyweight.
By the middle rounds, Parker had kept it close, causing problems for the favorite, who utilized a protective stance to edge the better periods.
The spectators' acceptance wore thin during constant clinching, with Parker going down multiple occasions during close exchanges.
The underdog finished the stronger, but many rounds were hard to judge. However, previous titleholder Carl Frampton - working with the broadcast network - had it significantly in favor for Parker, awarding Buatsi just only one stanza.
Buatsi progresses, but with the likes of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still appears challenging.
Hatton Memorial Fuels Manchester Derby Battle
In the secondary fight, Lyndon Arthur beat Rea by divided decision in a terrific local rivalry battle to secure the continental belt.
The stadium became quiet for a profoundly moving homage before the fight as 10 bells tolled in remembrance of local hero Hatton, who passed away at age 46 in September.
It was the initial significant broadcast boxing event in the city since Hatton's death. As the final chime faded, the silence transitioned to a shout of "only one Hatton".
The competition within the ring equaled the feeling. The eventual winner was knocked down by a left hook flush on the chin in the initial stage, getting up unsteadily to his footing on the eight count.
But the traditional red attire of Arthur soon advanced again. From the other corner, the opponent - in opposing blue - was injured soon, noticeable damage forming under his eye as the fighters traded punishing blows.
Rea, twenty-seven, landed power punches but Arthur, 34, fought fire with fire with stinging uppercuts as the action seldom decreased over 12 rounds.
Arthur barely secured the result with official scores of draw, close margin and clearer margin, but both fighters were determined and full of heart, everything their inspiration the legend represented.
