UK light heavyweight Buatsi secured a victory yet underwhelming start to life under new promoters with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a split decision in a close ten-round contest at the Manchester venue.
But Parker found success, landing some telling single shots in a match where the styles never quite clicked, with constant grabbing disrupting the contest.
The officials scored it 96-94 for to Buatsi, with the final judge a 95-95 draw.
"It wasn't my greatest performance but what do champions do? They win," Buatsi commented in the arena as he was faced disapproval from the crowd.
The Londoner got back to winning after his maiden career setback - an memorable fight with Callum Smith in last winter - to earn a twentieth professional win.
Several boxing experts thought the challenger, thirty-one, deserved the victory but the British boxer experienced the second setback of his 28-bout professional journey.
Buatsi Secures Victory in Slow Affair
The fighter's advancement has slowed in the past few years through management issues and considerable time of lack of fights.
Previously predicted as the rising talent of British boxing, he now finds himself trying to achieve expectations - possibly in what seems like a ultimate opportunity - under new promotional backing.
He began cautiously, with neither fighter ready to be aggressive in a tentative early stages.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in 2022 after damaging his hand in the contest, was making just his second appearance at light-heavyweight.
By the middle rounds, Parker had stayed in contention, creating difficulties for the favorite, who utilized a solid defense to barely take the cleaner stanzas.
The crowd's patience decreased because of frequent holding, with the challenger falling to the floor several times during clinches.
The underdog finished the stronger, but multiple periods were hard to judge. However, previous titleholder Carl Frampton - commentating on the broadcast network - had it particularly wide for the challenger, giving Buatsi just one round.
Buatsi progresses, but with the presence of hard hitter David Benavidez, and Russian champion Bivol leading the category, the journey to titles - judging by this showing - still appears challenging.
Hatton Memorial Motivates All-Manchester Contest
In the co-main event, Lyndon Arthur defeated Rea by split-decision in a outstanding local rivalry contest to capture the continental belt.
The venue fell silent for a deeply poignant tribute before the fight as a bell ceremony tolled in memory of the city's native Ricky Hatton, who succumbed at age 46 in last month.
It was the first major televised match card in the location since Hatton's death. As the concluding sound faded, the silence gave way to a chant of "only one Ricky Hatton".
The fighting within the ring matched the sentiment. The eventual winner was knocked down by a significant blow squarely on the chin in the initial stage, getting up unsteadily to his footing on the eight count.
But the red-colored attire of the fighter soon advanced again. Across the ring, Rea - in opposing blue - was marked up early, heavy swelling developing under his eye as the fighters exchanged punishing blows.
Rea, 27, landed power punches but the champion, 34, responded in kind with stinging uppercuts as the pace rarely dropped over twelve rounds.
Arthur narrowly won the decision with scoring tallies of 114-114, narrow lead and 115-112, but both fighters were relentless and full of heart, exactly what their hero the legend represented.
