British light-heavyweight Buatsi earned a win yet underwhelming start to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's activity level likely gained him a majority decision in a tight 10-round fight at the boxing arena.
But the challenger landed significant shots, delivering effective single shots in a contest where the styles failed to mesh, with excessive holding marring the fight.
Two judges had it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my greatest performance but what defines champions? They prevail," Buatsi stated in the ring as he was faced disapproval from the crowd.
The British fighter got back to winning after his initial pro loss - an thrilling contest with his previous opponent in last winter - to claim a 20th career victory.
Some ringside observers thought the challenger, 31, deserved the victory but the contender suffered the second loss of his 28-fight career.
The Champion Earns Win in Uneventful Match
The fighter's advancement has stalled in recent times through promotional disputes and extended periods of inactivity.
Once tipped as the next star of the British fight scene, he now stands trying to fulfil that early promise - possibly in what appears to be a final roll of the dice - under fresh management.
He began cautiously, with neither fighter willing to overcommit in a calculated initial rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after damaging his hand mid-fight, was making just his second fight at light heavyweight.
By the halfway stage, Parker had kept it close, making life awkward for the favorite, who worked behind a protective stance to edge the better rounds.
The audience's tolerance decreased amid frequent holding, with the challenger falling to the floor various instances during grappling.
The underdog finished the stronger, but several stanzas were challenging to assess. Nevertheless, ex-champion the boxing analyst - commentating on Dazn - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
Buatsi progresses, but with the caliber of power puncher David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the path to championship success - judging by this showing - still seems difficult.
Ricky Hatton Homage Fuels Manchester Derby Fight
In the supporting bout, Arthur overcame Rea by close verdict in a terrific Manchester vs Manchester battle to capture the continental championship.
The arena grew hushed for a deeply poignant tribute ahead of the contest as ten bells tolled in memory of local hero Ricky Hatton, who succumbed at age 46 in September.
It was the initial significant broadcast fight night in the city since Hatton's passing. As the last bell disappeared, the stillness became a roar of "only one Hatton".
The action within the ring reflected the emotion. Arthur was knocked down by a significant blow directly to the jaw in the early round, standing uncertainly to his feet on the referee's count.
But the traditional red attire of Arthur soon came forward again. From the other corner, Rea - in opposing blue - was marked up quickly, noticeable damage appearing under his face as the pair traded hard punches.
The challenger, 27, delivered effective strikes but the champion, thirty-four, matched the intensity with stinging uppercuts as the intensity seldom decreased over the full distance.
Arthur barely secured the verdict with judges scorecards of draw, 115-113 and wider gap, but both fighters were relentless and full of heart, everything their idol Hatton represented.
