British light-heavyweight Buatsi secured a victory yet unimpressive start to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a split decision in a tight 10-round fight at the Manchester venue.
But the challenger had his moments, connecting with powerful clean blows in a contest where the approaches failed to mesh, with excessive holding spoiling the fight.
The officials judged it 96-94 in favor to Buatsi, with the final judge a 95-95 draw.
"It wasn't my finest display but what defines champions? They win," the victor stated in the ringside as he was greeted with disapproval from the spectators.
The Londoner returned to winning ways after his first professional defeat - an thrilling contest with Callum Smith in last winter - to secure a 20th pro success.
Various analysts thought Parker, 31, should have been awarded the decision but the British boxer suffered the second loss of his 28-fight professional journey.
Buatsi Earns Win in Dull Encounter
The fighter's advancement has stalled in the past few years through promotional disputes and considerable time of inactivity.
Previously predicted as the next star of UK boxing, he now is positioned trying to realize his potential - perhaps in what appears to be a final roll of the dice - under fresh management.
He began cautiously, with the two boxers willing to overcommit in a tentative initial rounds.
Parker, whose single earlier setback came at the lower weight class against John Ryder in 2022 after suffering a hand injury during the bout, was participating in his second fight at the higher weight class.
By the halfway stage, the underdog had made it competitive, making life awkward for the favorite, who operated with a protective stance to barely take the more effective periods.
The spectators' acceptance decreased because of constant clinching, with the challenger going down various instances during grappling.
The underdog ended stronger, but many rounds were challenging to assess. Nevertheless, former world champion the boxing analyst - commentating on the broadcast network - had it particularly wide for Parker, awarding Buatsi just only one stanza.
Buatsi progresses, but with the caliber of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still seems difficult.
Ricky Hatton Memorial Motivates All-Manchester Fight
In the secondary fight, Arthur overcame Bradley Rea by divided decision in a outstanding local rivalry contest to capture the continental belt.
The stadium grew hushed for a extremely emotional tribute prior to the bout as 10 bells rang in honor of the city's native Ricky Hatton, who succumbed 46 years old in recently.
It was the premier televised boxing event in the city since Hatton's death. As the concluding sound disappeared, the quiet gave way to a roar of "there's just one Ricky Hatton".
The competition during the bout equaled the feeling. The eventual winner was knocked down by a significant blow directly to the jaw in the second round, rising shakily to his footing on the eight count.
But the Manchester United-red equipment of Arthur soon pressed ahead again. On the opposite side, Rea - in competitive blue - was marked up early, noticeable damage appearing under his eye as the duo traded hard punches.
Rea, twenty-seven, landed power punches but Arthur, thirty-four, fought fire with fire with effective uppercuts as the pace hardly diminished over twelve rounds.
The victor narrowly won the verdict with scoring tallies of draw, narrow lead and wider gap, but both fighters were determined and courageous, everything their idol the legend represented.
