British light heavyweight Buatsi earned a win yet underwhelming start to his career under recent management 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 tight ten-round fight at the Manchester venue.
But Parker found success, landing some telling single shots in a contest where the styles failed to mesh, with frequent clinching spoiling the action.
The scoring judges had it 96-94 in favor to the winner, with the final judge a 95-95.
"It wasn't my best showing but how do champions respond? They win," the victor commented in the arena as he was greeted with boos from the spectators.
The Londoner bounced back after his first professional defeat - an memorable fight with his previous opponent in last winter - to earn a 20th career victory.
Various analysts believed the challenger, thirty-one, deserved the win but the Derbyshire fighter experienced the second setback of his 28-bout career.
Buatsi Earns Win in Slow Encounter
Buatsi's progress has plateaued in the past few years through contract problems and extended periods of lack of fights.
Previously predicted as the next star of British boxing, he now finds himself trying to achieve expectations - perhaps in what feels like a last chance - under new promotional backing.
He began cautiously, with neither fighter willing to take risks in a tentative opening few rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after breaking his hand in the contest, was competing in only his second bout at light-heavyweight.
By the halfway stage, Parker had made it competitive, causing problems for the favorite, who worked behind a tight guard to barely take the better rounds.
The spectators' acceptance decreased during frequent holding, with Parker slipping to the canvas multiple occasions during close exchanges.
Parker closed well, but many rounds were challenging to assess. However, previous titleholder the boxing analyst - working with Dazn - had it particularly wide for Parker, scoring for Buatsi just a single round.
The victor advances, but with the presence of heavy hitter David Benavidez, and Russian champion Bivol ruling the division, the road to world glory - based on this performance - still seems difficult.
Hatton Homage Inspires Local Rivalry Battle
In the supporting bout, Arthur beat Rea by split-decision in a terrific local rivalry fight to win the European light heavyweight title.
The venue grew hushed for a extremely emotional tribute before the fight as ten bells tolled in remembrance of Manchester's own Ricky Hatton, who died 46 years old in last month.
It was the initial significant broadcast fight night in the location since Hatton's passing. As the last bell faded, the stillness gave way to a roar of "only one Hatton".
The fighting during the bout equaled the sentiment. The eventual winner was floored by a significant blow flush on the jaw in the second round, getting up unsteadily to his position on the count of eight.
But the red-colored shorts of Arthur soon advanced again. From the other corner, the opponent - in competitive blue - was marked up early, significant bruising appearing under his eye as the pair exchanged powerful shots.
The challenger, 27, delivered effective strikes but Arthur, thirty-four, responded in kind with powerful upward punches as the intensity hardly diminished over twelve rounds.
Arthur edged the decision with official scores of 114-114, 115-113 and wider gap, but the two athletes were determined and brave, everything their idol Hatton represented.
