UK light heavyweight Buatsi earned a win yet underwhelming beginning to life under new promoters with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a close ten-round bout at the Manchester venue.
But the challenger found success, delivering effective individual punches in a fight where the approaches failed to mesh, with frequent clinching marring the contest.
The officials judged it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my greatest display but what defines champions? They win," Buatsi said in the ringside as he was faced boos from the audience.
The British fighter got back to winning after his initial pro loss - an memorable fight with his previous opponent in February - to secure a 20th career victory.
Several boxing experts believed Parker, thirty-one, should have been awarded the victory but the contender suffered the second defeat of his 28-fight career.
The Champion Secures Victory in Dull Affair
Buatsi's progress has slowed in recent years through management issues and considerable time of lack of fights.
Earlier forecast as the rising talent of British boxing, he now finds himself trying to fulfil that early promise - possibly in what seems like a last chance - under recent support.
He began cautiously, with both competitors prepared to overcommit in a tentative initial rounds.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in 2022 after damaging his hand during the bout, was competing in only his second fight at light heavyweight.
By the halfway stage, Parker had stayed in contention, causing problems for Buatsi, who worked behind a protective stance to narrowly win the cleaner rounds.
The audience's tolerance wore thin amid regular grabbing, with the challenger going down various instances during clinches.
Parker finished the stronger, but several stanzas were difficult to score. However, ex-champion the boxing analyst - commentating on the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the likes of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the path to championship success - based on this performance - still seems difficult.
Hatton Homage Fuels All-Manchester Battle
In the secondary fight, Arthur beat Bradley Rea by divided decision in a terrific all-Manchester battle to capture the European light heavyweight belt.
The arena became quiet for a extremely emotional tribute prior to the bout as ten bells rang in memory of Manchester's own Ricky Hatton, who passed away aged 46 in last month.
It was the premier televised match card in the location since Hatton's death. As the final chime disappeared, the silence transitioned to a roar of "only one Hatton".
The fighting inside the ropes matched the emotion. Arthur was sent to the canvas by a powerful punch squarely on the jaw in the early round, rising shakily to his position on the count of eight.
But the Manchester United-red shorts of Arthur soon pressed ahead again. From the other corner, Rea - in opposing blue - was injured early, heavy swelling forming under his face as the fighters traded hard punches.
The challenger, twenty-seven, delivered effective strikes but Arthur, thirty-four, fought fire with fire with effective uppercuts as the pace seldom decreased over the full distance.
Arthur barely secured the result with scoring tallies of draw, narrow lead and wider gap, but the two athletes were relentless and full of heart, everything their hero Hatton embodied.
