British light-heavyweight Buatsi secured a victory yet unimpressive beginning to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a judges' verdict in a tight 10-round bout at the Co-op Live Arena.
But the challenger found success, connecting with powerful individual punches in a fight where the approaches didn't complement each other, with excessive holding marring the contest.
The scoring judges had it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my greatest showing but what defines champions? They win," the victor said in the arena as he was faced boos from the spectators.
The British fighter got back to winning after his first professional defeat - an memorable fight with his previous opponent in last winter - to secure a twentieth professional win.
Several boxing experts believed Parker, thirty-one, should have been awarded the win but the Derbyshire fighter experienced the second loss of his 28-fight career.
Buatsi Earns Win in Dull Encounter
His career development has stalled in recent years through contract problems and considerable time of inactivity.
Once tipped as the rising talent of British boxing, he now is positioned trying to fulfil that early promise - possibly in what seems like a ultimate opportunity - under fresh management.
He began cautiously, with both competitors willing to be aggressive in a cagey initial rounds.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in 2022 after damaging his hand in the contest, was competing in only his second appearance at light-heavyweight.
By the middle rounds, Parker had kept it close, causing problems for the favorite, who worked behind a protective stance to barely take the cleaner periods.
The crowd's patience diminished during regular grabbing, with the challenger going down various instances during close exchanges.
The underdog closed well, but many rounds were challenging to assess. Although, previous titleholder the boxing analyst - commentating on Dazn - had it strongly leaning for Parker, giving Buatsi just one round.
The victor advances, but with the likes of power puncher David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the path to championship success - judging by this showing - still seems difficult.
Hatton Tribute Motivates Manchester Derby Contest
In the supporting bout, Arthur beat Rea by divided decision in a outstanding all-Manchester fight to secure the European light-heavyweight belt.
The venue became quiet for a deeply poignant homage ahead of the contest as 10 bells rang in honor of Manchester's own Ricky Hatton, who succumbed aged 46 in recently.
It was the first major televised fight night in the city since Hatton's passing. As the final chime ended, the silence became a shout of "there's just one Ricky Hatton".
The action during the bout matched the feeling. Arthur was floored by a left hook directly to the face in the initial stage, getting up unsteadily to his feet on the eight count.
But the Manchester United-red shorts of the fighter soon advanced again. From the other corner, Rea - in opposing blue - was marked up early, heavy swelling appearing under his eye as the duo landed punishing blows.
The challenger, 27, delivered effective strikes but the champion, 34, matched the intensity with stinging uppercuts as the pace rarely dropped over 12 rounds.
Arthur narrowly won the verdict with official scores of draw, 115-113 and wider gap, but the two athletes were determined and brave, everything their idol the legend represented.
