UK light heavyweight Buatsi earned a win yet unimpressive debut to his career under recent management with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a close 10-round fight at the boxing arena.
But Parker had his moments, delivering effective clean blows in a contest where the techniques didn't complement each other, with constant grabbing disrupting the fight.
Two judges judged it 96-94 for to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my finest performance but how do champions respond? They prevail," Buatsi said in the arena as he was faced boos from the audience.
The British fighter got back to winning after his maiden career setback - an memorable fight with his previous opponent in last winter - to secure a 20th pro success.
Various analysts believed Parker, 31, merited the victory but the British boxer suffered the second setback of his 28-fight career.
The Champion Claims Triumph in Dull Affair
His career development has slowed in the past few years through contract problems and considerable time of inactivity.
Once tipped as the rising talent of UK boxing, he now finds himself trying to realize his potential - maybe in what appears to be a final roll of the dice - under fresh management.
He started carefully, with both competitors prepared to be aggressive in a calculated early stages.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was participating in his second bout at light-heavyweight.
By the middle rounds, Parker had stayed in contention, making life awkward for the favorite, who utilized a solid defense to narrowly win the cleaner periods.
The audience's tolerance decreased amid regular grabbing, with Parker going down several times during clinches.
The underdog closed well, but multiple periods were difficult to score. Nevertheless, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
The victor advances, but with the presence of heavy hitter David Benavidez, and Russian champion Bivol dominating the weight class, the journey to titles - based on this performance - still seems difficult.
Ricky Hatton Tribute Fuels Local Rivalry Contest
In the supporting bout, Arthur defeated Rea by split-decision in a excellent local rivalry contest to win the continental title.
The venue became quiet for a deeply poignant homage ahead of the contest as 10 bells tolled in memory of local hero Hatton, who died aged 46 in recently.
It was the premier televised boxing event in the location since Hatton's death. As the last bell faded, the silence became a shout of "only one Ricky Hatton".
The action during the bout reflected the emotion. Arthur was knocked down by a significant blow flush on the jaw in the early round, standing uncertainly to his position on the eight count.
But the Manchester United-red attire of the fighter soon pressed ahead again. Across the ring, Rea - in competitive blue - was marked up early, significant bruising forming under his orbital area as the fighters traded hard punches.
Rea, twenty-seven, connected with strong blows but Arthur, 34, fought fire with fire with stinging uppercuts as the pace seldom decreased over the full distance.
Arthur barely secured the verdict with judges scorecards of even score, 115-113 and wider gap, but both fighters were determined and brave, exactly what their inspiration the legend stood for.
