British light heavyweight Buatsi secured a victory yet unimpressive beginning to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a majority decision in a tight ten-round fight at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective individual punches in a fight where the techniques didn't complement each other, with excessive holding spoiling the fight.
The scoring judges had it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my finest performance but how do champions respond? They win," Buatsi said in the ring as he was faced disapproval from the audience.
The British fighter bounced back after his initial pro loss - an epic battle with Callum Smith in February - to claim a twentieth professional win.
Various analysts felt the challenger, thirty-one, should have been awarded the decision but the Derbyshire fighter endured the second defeat of his 28-bout career.
The Champion Secures Victory in Slow Encounter
Buatsi's progress has plateaued in recent times through promotional disputes and considerable time of inactivity.
Earlier forecast as the next star of the British fight scene, he now stands trying to achieve expectations - possibly in what appears to be a final roll of the dice - under new promotional backing.
He started carefully, with the two boxers ready to overcommit in a cagey initial rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after damaging his hand during the bout, was making just his second appearance at light-heavyweight.
By the fight's midpoint, Parker had kept it close, creating difficulties for the favorite, who utilized a protective stance to barely take the better stanzas.
The crowd's patience diminished amid regular grabbing, with the challenger going down various instances during clinches.
The underdog ended stronger, but multiple periods were hard to judge. Nevertheless, ex-champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for the challenger, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the likes of heavy hitter David Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - based on this performance - still seems difficult.
Hatton Tribute Inspires Manchester Derby Contest
In the supporting bout, Arthur beat Rea by close verdict in a excellent all-Manchester fight to secure the European light-heavyweight championship.
The arena grew hushed for a extremely emotional tribute ahead of the contest as ten bells tolled in memory of local hero Ricky Hatton, who passed away at age 46 in September.
It was the initial significant broadcast boxing event in the city since Hatton's death. As the concluding sound ended, the silence gave way to a roar of "there's only one Ricky Hatton".
The competition during the bout reflected the feeling. Arthur was knocked down by a significant blow flush on the chin in the initial stage, getting up unsteadily to his footing on the referee's count.
But the red-colored equipment of Arthur soon advanced again. On the opposite side, Rea - in competitive blue - was injured quickly, significant bruising developing under his face as the pair traded punishing blows.
The challenger, twenty-seven, connected with strong blows but Arthur, thirty-four, matched the intensity with stinging uppercuts as the action hardly diminished over 12 rounds.
Arthur barely secured the decision with judges scorecards of 114-114, narrow lead and clearer margin, but both fighters were relentless and full of heart, exactly what their hero Hatton represented.
