UK light-heavyweight Buatsi secured a victory yet disappointing debut to life under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a tight ten-round contest at the boxing arena.
But Parker had his moments, connecting with powerful clean blows in a match where the techniques failed to mesh, with excessive holding marring the action.
The officials judged it 96-94 to the winner, with the third a even score.
"It wasn't my finest display but what do champions do? They succeed," the victor commented in the arena as he was faced disapproval from the audience.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with his previous opponent in February - to claim a twentieth professional win.
Several boxing experts believed the challenger, thirty-one, merited the victory but the contender suffered the second loss of his 28-fight career.
The Champion Earns Triumph in Dull Match
His career development has stalled in recent years through promotional disputes and long spells of inactivity.
Previously predicted as the next star of UK boxing, he now finds himself trying to achieve expectations - possibly in what appears to be a final roll of the dice - under fresh management.
He began cautiously, with neither fighter prepared to overcommit in a calculated opening few rounds.
Parker, whose sole prior loss came at the lower weight class against John Ryder in 2022 after damaging his hand during the bout, was making just his second bout at the higher weight class.
By the middle rounds, Parker had kept it close, making life awkward for the favorite, who operated with a solid defense to barely take the more effective rounds.
The spectators' acceptance decreased amid constant clinching, with Parker going down various instances during clinches.
Parker finished the stronger, but multiple periods were hard to judge. Nevertheless, ex-champion the boxing analyst - commentating on the broadcast network - had it particularly wide for the challenger, scoring for Buatsi just one round.
Buatsi moves on, but with the presence of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the road to world glory - based on this performance - still looks steep.
Hatton Tribute Fuels All-Manchester Battle
In the secondary fight, Arthur defeated Rea by close verdict in a excellent all-Manchester battle to capture the European light-heavyweight belt.
The stadium grew hushed for a extremely emotional memorial ahead of the contest as a bell ceremony tolled in honor of the city's native Ricky Hatton, who succumbed 46 years old in September.
It was the initial significant broadcast fight night in the urban center since Hatton's passing. As the last bell faded, the silence became a roar of "there's only one Ricky Hatton".
The action inside the ropes equaled the feeling. The eventual winner was knocked down by a left hook directly to the jaw in the initial stage, standing uncertainly to his footing on the eight count.
But the Manchester United-red attire of the fighter soon pressed ahead again. On the opposite side, the opponent - in rival City blue - was injured quickly, noticeable damage developing under his face as the fighters landed punishing blows.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, fought fire with fire with stinging uppercuts as the intensity seldom decreased over the full distance.
Arthur edged the decision with official scores of draw, narrow lead and clearer margin, but both fighters were persistent and full of heart, all the qualities their idol the legend stood for.
