UK light-heavyweight Buatsi made a winning yet underwhelming start to his career under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's work rate seemingly secured him a judges' verdict in a close 10-round fight at the boxing arena.
But Parker had his moments, landing some telling single shots in a fight where the approaches never quite clicked, with frequent clinching spoiling the contest.
Two judges judged it 96-94 in favor to Buatsi, with the final judge a 95-95 draw.
"It wasn't my greatest display but what do champions do? They succeed," Buatsi said in the ring as he was greeted with disapproval from the spectators.
The Londoner bounced back after his maiden career setback - an memorable fight with Callum Smith in last winter - to secure a 20th career victory.
Various analysts thought the challenger, thirty-one, should have been awarded the win but the British boxer endured the second loss of his 28-match career.
The Champion Secures Win in Slow Affair
The fighter's advancement has stalled in recent years through management issues and considerable time of inactivity.
Earlier forecast as the future of British boxing, he now is positioned trying to fulfil that early promise - maybe in what appears to be a final roll of the dice - under recent support.
He opened tentatively, with the two boxers willing to be aggressive in a calculated initial rounds.
Parker, whose sole prior loss 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 halfway stage, Parker had kept it close, causing problems for the favorite, who worked behind a protective stance to barely take the better periods.
The spectators' acceptance decreased during frequent holding, with the challenger slipping to the canvas multiple occasions during close exchanges.
Parker finished the stronger, but many rounds were hard to judge. Although, former world champion the boxing analyst - commentating on Dazn - had it particularly wide for Parker, giving Buatsi just one round.
The victor progresses, but with the likes of power puncher Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still looks steep.
Hatton Tribute Fuels Manchester Derby Fight
In the supporting bout, Lyndon Arthur defeated Bradley Rea by close verdict in a excellent Manchester vs Manchester contest to secure the continental title.
The arena grew hushed for a profoundly moving tribute prior to the bout as 10 bells tolled in honor of the city's native Ricky Hatton, who died 46 years old in last month.
It was the premier televised match card in the location since Hatton's death. As the final chime disappeared, the silence became a shout of "there's just one Hatton".
The competition during the bout reflected the sentiment. The eventual winner was knocked down by a powerful punch squarely on the face in the initial stage, rising shakily to his footing on the count of eight.
But the Manchester United-red shorts of Arthur soon advanced again. On the opposite side, Rea - in competitive blue - was injured quickly, noticeable damage developing under his face as the pair landed powerful shots.
Rea, twenty-seven, connected with strong blows but the champion, 34, matched the intensity with effective uppercuts as the intensity hardly diminished over twelve rounds.
The victor edged the result with judges scorecards of 114-114, narrow lead and 115-112, but both fighters were persistent and brave, exactly what their hero the legend embodied.
