British light-heavyweight Buatsi secured a victory yet disappointing debut to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a majority decision in a competitive ten-round fight at the Co-op Live Arena.
But Parker found success, landing some telling clean blows in a fight where the approaches didn't complement each other, with excessive holding spoiling the action.
The officials judged it 96-94 in favor to the winner, with the remaining official a 95-95 draw.
"It wasn't my best display but how do champions respond? They prevail," the victor said in the ringside as he was greeted with boos from the crowd.
The Londoner got back to winning after his initial pro loss - an epic battle with Callum Smith in February - to earn a twentieth professional win.
Some ringside observers thought Parker, 31, deserved the win but the Derbyshire fighter suffered the second defeat of his 28-match professional journey.
Buatsi Claims Victory in Slow Affair
Buatsi's progress has slowed in recent times through contract problems and long spells of inactivity.
Earlier forecast as the next star of the British fight scene, he now is positioned trying to fulfil that early promise - possibly in what appears to be a final roll of the dice - under new promotional backing.
He started carefully, with the two boxers ready to take risks in a tentative initial rounds.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in two years ago after suffering a hand injury mid-fight, was participating in his second fight at the higher weight class.
By the middle rounds, Parker had kept it close, causing problems for the favorite, who utilized a tight guard to edge the more effective stanzas.
The spectators' acceptance wore thin during constant clinching, with the challenger going down multiple occasions during grappling.
Parker closed well, but many rounds were challenging to assess. Although, former world champion Carl Frampton - commentating on Dazn - had it particularly wide for the challenger, awarding Buatsi just a single round.
Buatsi moves on, but with the caliber of hard hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the path to championship success - judging by this showing - still looks steep.
Hatton Homage Inspires Manchester Derby Fight
In the secondary fight, Arthur beat Rea by close verdict in a terrific Manchester vs Manchester fight to capture the continental belt.
The stadium grew hushed for a profoundly moving homage prior to the bout as 10 bells rang in memory of Manchester's own Ricky Hatton, who passed away 46 years old in September.
It was the initial significant broadcast boxing event in the city since Hatton's death. As the last bell faded, the silence transitioned to a chant of "only one Ricky Hatton".
The competition within the ring equaled the emotion. Arthur was sent to the canvas by a significant blow flush on the chin in the early round, getting up unsteadily to his position on the count of eight.
But the Manchester United-red equipment of Arthur soon advanced again. On the opposite side, the opponent - in opposing blue - was injured quickly, noticeable damage developing under his face as the pair landed punishing blows.
The challenger, twenty-seven, landed power punches but the champion, 34, responded in kind with effective uppercuts as the action rarely dropped over twelve rounds.
Arthur edged the decision with official scores of draw, close margin and wider gap, but both fighters were determined and full of heart, everything their hero Hatton represented.
