UK light-heavyweight Buatsi earned a win yet disappointing start to life under new promoters with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a close 10-round bout at the Manchester venue.
But the challenger had his moments, landing some telling individual punches in a contest where the techniques failed to mesh, with excessive holding disrupting the action.
The officials had it 96-94 for to the winner, with the remaining official a even score.
"It wasn't my best performance but how do champions respond? They prevail," Buatsi said in the ringside as he was greeted with boos from the crowd.
The Londoner got back to winning after his first professional defeat - an memorable fight with Callum Smith in February - to secure a 20th pro success.
Various analysts felt the challenger, thirty-one, merited the victory but the contender suffered the second loss of his 28-bout career.
The Champion Claims Triumph in Slow Encounter
The fighter's advancement has stalled in the past few years through management issues and extended periods of inactivity.
Once tipped as the rising talent of British boxing, he now finds himself trying to fulfil that early promise - maybe in what feels like a final roll of the dice - under fresh management.
He began cautiously, with both competitors prepared to be aggressive in a tentative opening few rounds.
Parker, whose single earlier setback came at super-middleweight against John Ryder in 2022 after breaking his hand in the contest, was making just his second bout at light heavyweight.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for Buatsi, who utilized a tight guard to barely take the better periods.
The crowd's patience diminished amid regular grabbing, with the challenger falling to the floor multiple occasions during grappling.
Parker closed well, but several stanzas were hard to judge. However, ex-champion Carl Frampton - working with Dazn - had it strongly leaning for the challenger, awarding Buatsi just a single round.
The victor advances, but with the likes of hard hitter David Benavidez, and Russian star Bivol leading the category, the journey to titles - based on this performance - still seems difficult.
Hatton Memorial Fuels Manchester Derby Contest
In the supporting bout, Lyndon Arthur beat Bradley Rea by split-decision in a outstanding all-Manchester fight to win the continental title.
The stadium grew hushed for a profoundly moving memorial before the fight as 10 bells tolled in honor of local hero Hatton, who succumbed at age 46 in September.
It was the initial significant broadcast match card in the location since the passing of Hatton. As the last bell ended, the stillness transitioned to a shout of "there's just one Hatton".
The competition during the bout equaled the feeling. The eventual winner was floored by a powerful punch directly to the face in the initial stage, getting up unsteadily to his position on the eight count.
But the traditional red equipment of the fighter soon pressed ahead again. From the other corner, Rea - in competitive blue - was injured quickly, heavy swelling appearing under his eye as the pair landed powerful shots.
Rea, 27, connected with strong blows but the champion, 34, matched the intensity with powerful upward punches as the pace seldom decreased over 12 rounds.
The victor narrowly won the verdict with official scores of draw, narrow lead and wider gap, but each competitor were determined and courageous, everything their hero the legend embodied.
