British light-heavyweight Buatsi made a winning yet disappointing beginning to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a majority decision in a tight 10-round bout at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful single shots in a contest where the approaches failed to mesh, with excessive holding spoiling the fight.
The officials judged it 96-94 to Buatsi, with the third a even score.
"It wasn't my best showing but how do champions respond? They prevail," Buatsi said in the ring as he was faced negative reactions from the crowd.
The British fighter bounced back after his initial pro loss - an memorable fight with Callum Smith in February - to claim a twentieth professional win.
Several boxing experts thought Parker, thirty-one, deserved the win but the Derbyshire fighter endured the second loss of his 28-bout professional journey.
The Champion Claims Victory in Dull Affair
Buatsi's progress has slowed in recent years through management issues and long spells of inactivity.
Previously predicted as the rising talent of UK boxing, he now finds himself trying to realize his potential - perhaps in what feels like a last chance - under new promotional backing.
He started carefully, with both competitors ready to take risks in a cagey opening few rounds.
Parker, whose single earlier setback came at the lower weight class against John Ryder in 2022 after breaking his hand in the contest, was participating in his second appearance at light-heavyweight.
By the halfway stage, Parker had stayed in contention, making life awkward for the favorite, who operated with a protective stance to barely take the cleaner stanzas.
The audience's tolerance decreased during frequent holding, with the challenger falling to the floor multiple occasions during grappling.
Parker closed well, but several stanzas were difficult to score. Although, previous titleholder Carl Frampton - working with the broadcast network - had it particularly wide for Parker, awarding Buatsi just a single round.
The victor progresses, but with the presence of power puncher Benavidez, and Russian star Bivol dominating the weight class, the path to championship success - based on this performance - still appears challenging.
Ricky Hatton Tribute Fuels Local Rivalry Fight
In the supporting bout, Lyndon Arthur overcame Rea by split-decision in a terrific all-Manchester battle to capture the European light-heavyweight title.
The stadium fell silent for a profoundly moving memorial prior to the bout as a bell ceremony sounded in remembrance of the city's native Hatton, who passed away 46 years old in September.
It was the premier televised match card in the city since Hatton's death. As the final chime disappeared, the silence became a chant of "only one Hatton".
The fighting inside the ropes reflected the emotion. The eventual winner was knocked down by a left hook directly to the chin in the second round, rising shakily to his position on the eight count.
But the Manchester United-red equipment of the fighter soon came forward again. On the opposite side, the opponent - in competitive blue - was damaged quickly, heavy swelling appearing under his orbital area as the pair exchanged punishing blows.
Rea, twenty-seven, connected with strong blows but Arthur, 34, responded in kind with stinging uppercuts as the pace rarely dropped over 12 rounds.
Arthur narrowly won the result with official scores of even score, 115-113 and wider gap, but the two athletes were determined and courageous, everything their hero the legend stood for.
