UK light-heavyweight Buatsi made a winning yet underwhelming debut to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a split decision in a tight 10-round bout at the boxing arena.
But the challenger had his moments, landing some telling single shots in a fight where the styles never quite clicked, with constant grabbing marring the contest.
The officials judged it 96-94 in favor to the winner, with the final judge a 95-95.
"It wasn't my greatest performance but what defines champions? They succeed," Buatsi said in the ringside as he was faced boos from the spectators.
The British fighter returned to winning ways after his first professional defeat - an memorable fight with his previous opponent in February - to earn a 20th pro success.
Some ringside observers believed Parker, thirty-one, deserved the decision but the Derbyshire fighter endured the second setback of his 28-match professional journey.
The Champion Secures Triumph in Slow Affair
The fighter's advancement has plateaued in recent times through promotional disputes and considerable time of lack of fights.
Once tipped as the next star of British boxing, he now is positioned trying to fulfil that early promise - possibly in what appears to be a ultimate opportunity - under new promotional backing.
He opened tentatively, with the two boxers willing to overcommit in a tentative initial rounds.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in 2022 after damaging his hand in the contest, was competing in only his second bout at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, causing problems for the favorite, who worked behind a solid defense to barely take the cleaner periods.
The spectators' acceptance decreased amid frequent holding, with the challenger going down multiple occasions during clinches.
Parker finished the stronger, but many rounds were challenging to assess. Nevertheless, ex-champion Carl Frampton - providing analysis for Dazn - had it particularly wide for Parker, scoring for Buatsi just a single round.
The victor advances, but with the likes of heavy hitter David Benavidez, and Russian champion Bivol ruling the division, the path to championship success - judging by this showing - still seems difficult.
Ricky Hatton Tribute Fuels Local Rivalry Fight
In the secondary fight, Lyndon Arthur beat Bradley Rea by divided decision in a excellent Manchester vs Manchester battle to capture the European light-heavyweight belt.
The stadium became quiet for a profoundly moving tribute prior to the bout as 10 bells tolled in honor of local hero Ricky Hatton, who succumbed aged 46 in last month.
It was the initial significant broadcast match card in the city since the passing of Hatton. As the final chime faded, the quiet gave way to a roar of "only one Hatton".
The fighting within the ring equaled the emotion. The eventual winner was sent to the canvas by a left hook squarely on the chin in the second round, getting up unsteadily to his position on the count of eight.
But the red-colored attire of the fighter soon advanced again. On the opposite side, the opponent - in competitive blue - was damaged early, noticeable damage forming under his orbital area as the duo exchanged punishing blows.
The challenger, twenty-seven, connected with strong blows but the champion, 34, fought fire with fire with powerful upward punches as the intensity rarely dropped over 12 rounds.
Arthur barely secured the verdict with scoring tallies of draw, narrow lead and wider gap, but both fighters were persistent and courageous, all the qualities their hero the legend represented.
