UK light-heavyweight Buatsi made a winning yet unimpressive debut to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a competitive 10-round contest at the Manchester venue.
But the challenger had his moments, landing some telling individual punches in a contest where the styles never quite clicked, with excessive holding disrupting the fight.
The scoring judges judged it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my best showing but what defines champions? They prevail," the victor said in the ringside as he was greeted with negative reactions from the audience.
The British fighter bounced back after his first professional defeat - an thrilling contest with his previous opponent in last winter - to earn a 20th career victory.
Various analysts thought Parker, 31, should have been awarded the decision but the Derbyshire fighter endured the second defeat of his 28-bout professional journey.
Buatsi Secures Win in Uneventful Match
His career development has stalled in the past few years through promotional disputes and extended periods of lack of fights.
Once tipped as the next star of UK boxing, he now is positioned trying to achieve expectations - perhaps in what appears to be a last chance - under recent support.
He opened tentatively, with both competitors willing to take risks in a cagey early stages.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in two years ago after suffering a hand injury in the contest, was competing in only his second bout at light-heavyweight.
By the fight's midpoint, the underdog had made it competitive, creating difficulties for Buatsi, who utilized a protective stance to narrowly win the cleaner periods.
The spectators' acceptance wore thin because of constant clinching, with the challenger slipping to the canvas multiple occasions during clinches.
The underdog ended stronger, but several stanzas were challenging to assess. Although, former world champion Carl Frampton - providing analysis for Dazn - had it particularly wide for the challenger, giving Buatsi just only one stanza.
The victor advances, but with the likes of power puncher David Benavidez, and Russian star Dmitry Bivol leading the category, the journey to titles - on this evidence - still looks steep.
Hatton Homage Fuels All-Manchester Contest
In the secondary fight, Arthur overcame Rea by close verdict in a excellent local rivalry contest to win the European light-heavyweight championship.
The arena grew hushed for a profoundly moving memorial ahead of the contest as ten bells rang in honor of the city's native Ricky Hatton, who passed away aged 46 in last month.
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 shout of "there's only one Hatton".
The action inside the ropes matched the feeling. Arthur was sent to the canvas by a significant blow flush on the face in the second round, standing uncertainly to his footing on the referee's count.
But the Manchester United-red shorts of the fighter soon pressed ahead again. On the opposite side, Rea - in rival City blue - was marked up quickly, heavy swelling forming under his orbital area as the fighters landed powerful shots.
The challenger, 27, delivered effective strikes but Arthur, thirty-four, matched the intensity with effective uppercuts as the intensity hardly diminished over the full distance.
The victor narrowly won the decision with scoring tallies of even score, 115-113 and 115-112, but each competitor were persistent and full of heart, all the qualities their hero the legend stood for.
