UK light heavyweight Buatsi secured a victory yet underwhelming beginning to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level likely gained him a majority decision in a close ten-round contest at the Manchester venue.
But Parker landed significant shots, landing some telling individual punches in a fight where the styles failed to mesh, with constant grabbing spoiling the fight.
The scoring judges judged it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my greatest display but what defines champions? They succeed," the victor said in the arena as he was greeted with boos from the crowd.
The British fighter got back to winning after his initial pro loss - an epic battle with his previous opponent in February - to claim a twentieth professional win.
Several boxing experts felt Parker, thirty-one, should have been awarded the decision but the Derbyshire fighter endured the second setback of his 28-bout professional journey.
Buatsi Earns Victory in Slow Affair
His career development has plateaued in recent years through promotional disputes and long spells of lack of fights.
Previously predicted as the future of British boxing, he now is positioned trying to fulfil that early promise - perhaps in what feels like a final roll of the dice - under new promotional backing.
He opened tentatively, with neither fighter willing to take risks in a tentative opening few rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in two years ago after breaking his hand in the contest, was competing in only his second fight at light-heavyweight.
By the middle rounds, the underdog had made it competitive, causing problems for Buatsi, who worked behind a protective stance to barely take the cleaner periods.
The spectators' acceptance diminished amid constant clinching, with the challenger going down various instances during close exchanges.
Parker closed well, but many rounds were hard to judge. Nevertheless, previous titleholder Carl Frampton - providing analysis for Dazn - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
Buatsi progresses, but with the likes of power puncher Benavidez, and Russian star Bivol ruling the division, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Memorial Inspires Manchester Derby Battle
In the secondary fight, Arthur beat Bradley Rea by close verdict in a excellent all-Manchester contest to secure the European light heavyweight championship.
The venue became quiet for a deeply poignant homage before the fight as a bell ceremony rang in memory of the city's native Ricky Hatton, who succumbed at age 46 in recently.
It was the premier televised match card in the location since the passing of Hatton. As the concluding sound disappeared, the quiet transitioned to a chant of "there's only one Hatton".
The competition during the bout reflected the sentiment. Arthur was knocked down by a left hook directly to the chin in the second round, getting up unsteadily to his position on the count of eight.
But the red-colored attire of the fighter soon came forward again. From the other corner, the opponent - in competitive blue - was injured quickly, noticeable damage developing under his orbital area as the fighters exchanged powerful shots.
Rea, 27, landed power punches but Arthur, 34, fought fire with fire with stinging uppercuts as the intensity rarely dropped over the full distance.
Arthur edged the verdict with scoring tallies of even score, narrow lead and wider gap, but the two athletes were determined and brave, exactly what their idol Hatton embodied.
