UK light heavyweight Buatsi earned a win yet underwhelming beginning to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a tight 10-round contest at the Manchester venue.
But Parker had his moments, delivering effective single shots in a fight where the techniques didn't complement each other, with frequent clinching marring the action.
The officials scored it 96-94 for to the winner, with the final judge a 95-95 draw.
"It wasn't my finest showing but what do champions do? They prevail," the victor commented in the ring as he was faced disapproval from the spectators.
The British fighter returned to winning ways after his maiden career setback - an epic battle with his previous opponent in February - to earn a 20th career victory.
Several boxing experts believed the challenger, thirty-one, should have been awarded the win but the contender suffered the second loss of his 28-bout career.
Buatsi Claims Triumph in Dull Affair
The fighter's advancement has plateaued in recent times through promotional disputes and long spells of inactivity.
Earlier forecast as the rising talent of the British fight scene, 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 be aggressive in a calculated initial rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after breaking his hand during the bout, was making just his second fight at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for the favorite, who utilized a protective stance to edge the better stanzas.
The spectators' acceptance decreased during frequent holding, with the challenger falling to the floor multiple occasions during clinches.
The underdog ended stronger, but multiple periods were difficult to score. Nevertheless, ex-champion Carl Frampton - working with the broadcast network - had it significantly in favor for the challenger, giving Buatsi just a single round.
The victor progresses, but with the caliber of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still appears challenging.
Hatton Memorial Inspires All-Manchester Battle
In the co-main event, Arthur overcame Rea by close verdict in a excellent Manchester vs Manchester battle to secure the European light heavyweight belt.
The stadium became quiet for a extremely emotional tribute prior to the bout as ten bells sounded in honor of local hero Hatton, who succumbed aged 46 in last month.
It was the premier televised match card in the urban center since the passing of Hatton. As the concluding sound faded, the silence transitioned to a roar of "only one Hatton".
The fighting during the bout matched the sentiment. Arthur was sent to the canvas by a powerful punch directly to the face in the second round, standing uncertainly to his feet on the referee's count.
But the red-colored shorts of Arthur soon pressed ahead again. From the other corner, Rea - in opposing blue - was marked up early, heavy swelling appearing under his eye as the fighters exchanged powerful shots.
The challenger, 27, landed power punches but the champion, 34, responded in kind with effective uppercuts as the intensity hardly diminished over twelve rounds.
Arthur edged the result with official scores of draw, 115-113 and clearer margin, but each competitor were relentless and full of heart, everything their hero Hatton embodied.
