UK light heavyweight Buatsi secured a victory yet disappointing debut to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a judges' verdict in a close ten-round bout at the boxing arena.
But Parker found success, connecting with powerful single shots in a match where the techniques failed to mesh, with constant grabbing marring the fight.
Two judges had it 96-94 for to Buatsi, with the third a 95-95 draw.
"It wasn't my finest showing but how do champions respond? They prevail," the victor stated in the arena as he was met by negative reactions from the audience.
The Londoner got back to winning after his maiden career setback - an memorable fight with Callum Smith in February - to secure a 20th pro success.
Some ringside observers believed Parker, thirty-one, should have been awarded the decision but the British boxer suffered the second defeat of his 28-match career.
Buatsi Claims Win in Slow Affair
Buatsi's progress has slowed in the past few years through management issues and considerable time of inactivity.
Earlier forecast as the future of UK boxing, he now is positioned trying to realize his potential - perhaps in what seems like a last chance - under fresh management.
He started carefully, with both competitors prepared to take risks in a calculated initial rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after suffering a hand injury during the bout, was making just his second fight at the higher weight class.
By the halfway stage, the underdog had made it competitive, causing problems for the favorite, who worked behind a solid defense to barely take the cleaner rounds.
The audience's tolerance decreased because of regular grabbing, with Parker slipping to the canvas multiple occasions during close exchanges.
Parker ended stronger, but several stanzas were difficult to score. Although, previous titleholder Carl Frampton - providing analysis for Dazn - had it particularly wide for Parker, giving Buatsi just one round.
Buatsi advances, but with the likes of heavy hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the path to championship success - based on this performance - still appears challenging.
Ricky Hatton Tribute Fuels Manchester Derby Contest
In the co-main event, Arthur beat Rea by close verdict in a excellent local rivalry contest to win the continental belt.
The arena fell silent for a extremely emotional memorial before the fight as ten bells tolled in remembrance of Manchester's own Ricky Hatton, who passed away at age 46 in recently.
It was the initial significant broadcast match card in the urban center since Hatton's death. As the concluding sound disappeared, the stillness transitioned to a roar of "there's just one Hatton".
The fighting inside the ropes matched the sentiment. Arthur was knocked down by a powerful punch squarely on the jaw in the early round, standing uncertainly to his feet on the referee's count.
But the traditional red attire of Arthur soon advanced again. On the opposite side, Rea - in rival City blue - was damaged soon, heavy swelling forming under his eye as the fighters landed hard punches.
Rea, twenty-seven, connected with strong blows but the champion, 34, fought fire with fire with powerful upward punches as the pace hardly diminished over 12 rounds.
Arthur edged the result with judges scorecards of draw, close margin and wider gap, but the two athletes were persistent and full of heart, everything their inspiration the legend stood for.
