British light-heavyweight Buatsi made a winning yet disappointing beginning to his career under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a majority decision in a competitive 10-round bout at the Manchester venue.
But Parker had his moments, landing some telling clean blows in a fight where the styles failed to mesh, with excessive holding spoiling the action.
The scoring judges scored it 96-94 to Buatsi, with the remaining official a even score.
"It wasn't my finest display but what do champions do? They succeed," Buatsi commented in the ringside as he was met by boos from the crowd.
The Londoner got back to winning after his first professional defeat - an memorable fight with his previous opponent in February - to secure a 20th career victory.
Various analysts felt the challenger, thirty-one, should have been awarded the decision but the British boxer suffered the second defeat of his 28-bout career.
Buatsi Claims Triumph in Slow Affair
The fighter's advancement has stalled in recent years through promotional disputes and considerable time of inactivity.
Once tipped as the rising talent of UK boxing, he now is positioned trying to fulfil that early promise - perhaps in what feels like a final roll of the dice - under fresh management.
He opened tentatively, with neither fighter willing to be aggressive in a tentative opening few rounds.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in 2022 after suffering a hand injury in the contest, was participating in his second fight at light heavyweight.
By the fight's midpoint, the underdog had made it competitive, making life awkward for the favorite, who worked behind a protective stance to barely take the cleaner periods.
The audience's tolerance decreased because of constant clinching, with Parker going down multiple occasions during grappling.
The underdog ended stronger, but multiple periods were challenging to assess. However, previous titleholder Carl Frampton - providing analysis for Dazn - had it particularly wide for Parker, awarding Buatsi just one round.
The victor moves on, but with the caliber of heavy hitter David Benavidez, and Russian champion Dmitry Bivol ruling the division, the journey to titles - on this evidence - still appears challenging.
Hatton Memorial Inspires All-Manchester Contest
In the co-main event, Lyndon Arthur overcame Bradley Rea by close verdict in a excellent local rivalry fight to capture the continental title.
The stadium became quiet for a extremely emotional memorial prior to the bout as ten bells rang in memory of the city's native Ricky Hatton, who succumbed aged 46 in last month.
It was the first major televised fight night in the city since Hatton's passing. As the last bell disappeared, the silence gave way to a roar of "only one Ricky Hatton".
The fighting within the ring equaled the feeling. The eventual winner was sent to the canvas by a significant blow directly to the face in the second round, getting up unsteadily to his footing on the count of eight.
But the traditional red shorts of Arthur soon pressed ahead again. Across the ring, the opponent - in rival City blue - was marked up early, noticeable damage appearing under his face as the fighters exchanged hard punches.
The challenger, twenty-seven, delivered effective strikes but the champion, 34, responded in kind with powerful upward punches as the action hardly diminished over twelve rounds.
The victor edged the result with judges scorecards of 114-114, close margin and wider gap, but each competitor were persistent and courageous, exactly what their idol the legend embodied.
