UK light-heavyweight Buatsi made a winning yet underwhelming start to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate seemingly secured him a majority decision in a tight ten-round contest at the boxing arena.
But Parker landed significant shots, landing some telling single shots in a contest where the techniques failed to mesh, with frequent clinching marring the contest.
The scoring judges judged it 96-94 for to the winner, with the final judge a even score.
"It wasn't my finest performance but what defines champions? They succeed," the victor said in the arena as he was met by boos from the crowd.
The British fighter returned to winning ways after his first professional defeat - an thrilling contest with Callum Smith in last winter - to earn a twentieth professional win.
Some ringside observers thought Parker, 31, deserved the win but the Derbyshire fighter endured the second loss of his 28-fight professional journey.
The Champion Claims Victory in Slow Affair
The fighter's advancement has slowed in the past few years through management issues and extended periods of inactivity.
Earlier forecast as the future of British boxing, he now is positioned trying to fulfil that early promise - perhaps in what appears to be a last chance - under new promotional backing.
He began cautiously, with both competitors prepared to overcommit in a cagey initial rounds.
Parker, whose single earlier setback came at super-middleweight against John Ryder in 2022 after damaging his hand in the contest, was participating in his second appearance at light heavyweight.
By the middle rounds, the underdog had made it competitive, making life awkward for Buatsi, who worked behind a protective stance to barely take the cleaner periods.
The spectators' acceptance decreased during regular grabbing, with the challenger going down various instances during clinches.
Parker finished the stronger, but several stanzas were hard to judge. However, previous titleholder the boxing analyst - commentating on the broadcast network - had it particularly wide for the challenger, giving Buatsi just one round.
Buatsi progresses, but with the presence of heavy hitter Benavidez, and Eastern European fighter Bivol leading the category, the path to championship success - based on this performance - still appears challenging.
Hatton Tribute Fuels Manchester Derby Fight
In the co-main event, Lyndon Arthur beat Bradley Rea by split-decision in a outstanding local rivalry battle to secure the European light heavyweight belt.
The venue fell silent for a extremely emotional memorial before the fight as a bell ceremony tolled in honor of the city's native Ricky Hatton, who died 46 years old in September.
It was the premier televised boxing event in the location since the passing of Hatton. As the concluding sound ended, the stillness transitioned to a roar of "only one Hatton".
The competition inside the ropes matched the emotion. Arthur was sent to the canvas by a left hook flush on the chin in the early round, getting up unsteadily to his feet on the referee's count.
But the red-colored equipment of Arthur soon came forward again. Across the ring, Rea - in rival City blue - was marked up early, significant bruising developing under his orbital area as the pair landed hard punches.
Rea, twenty-seven, delivered effective strikes but the champion, 34, fought fire with fire with stinging uppercuts as the pace hardly diminished over twelve rounds.
Arthur barely secured the verdict with scoring tallies of even score, close margin and wider gap, but both fighters were determined and brave, everything their hero Hatton stood for.
