UK light heavyweight Buatsi made a winning yet unimpressive beginning to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a split decision in a close ten-round fight at the Co-op Live Arena.
But Parker had his moments, landing some telling individual punches in a match where the approaches failed to mesh, with constant grabbing marring the fight.
The scoring judges had it 96-94 in favor to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my finest showing but what do champions do? They win," the victor said in the arena as he was faced disapproval from the crowd.
The British fighter returned to winning ways after his first professional defeat - an memorable fight with his previous opponent in last winter - to secure a twentieth professional win.
Various analysts believed Parker, thirty-one, should have been awarded the win but the Derbyshire fighter experienced the second setback of his 28-bout career.
Buatsi Earns Victory in Dull Match
The fighter's advancement has slowed in recent years through promotional disputes and extended periods of inactivity.
Earlier forecast as the rising talent of British boxing, he now is positioned trying to realize his potential - perhaps in what feels like a final roll of the dice - under new promotional backing.
He opened tentatively, with both competitors prepared to be aggressive in a calculated opening few rounds.
The challenger, whose only previous defeat 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 fight at light heavyweight.
By the middle rounds, the underdog had made it competitive, causing problems for the favorite, who operated with a solid defense to edge the more effective periods.
The crowd's patience decreased because of regular grabbing, with Parker falling to the floor several times during grappling.
Parker finished the stronger, but many rounds were hard to judge. Nevertheless, previous titleholder Carl Frampton - working with Dazn - had it particularly wide for Parker, giving Buatsi just a single round.
The victor progresses, but with the presence of power puncher Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - on this evidence - still looks steep.
Hatton Memorial Fuels Local Rivalry Fight
In the co-main event, Arthur beat Bradley Rea by divided decision in a terrific local rivalry contest to win the continental title.
The arena grew hushed for a deeply poignant memorial before the fight as ten bells sounded in remembrance of Manchester's own Ricky Hatton, who succumbed aged 46 in last month.
It was the initial significant broadcast fight night in the city since Hatton's death. As the concluding sound faded, the stillness transitioned to a shout of "there's only one Ricky Hatton".
The competition inside the ropes matched the sentiment. The eventual winner was knocked down by a powerful punch flush on the jaw in the initial stage, getting up unsteadily to his footing on the count of eight.
But the Manchester United-red attire of Arthur soon advanced again. From the other corner, Rea - in competitive blue - was marked up soon, heavy swelling appearing under his orbital area as the duo exchanged powerful shots.
The challenger, 27, connected with strong blows but Arthur, thirty-four, responded in kind with stinging uppercuts as the pace seldom decreased over twelve rounds.
Arthur barely secured the result with official scores of draw, close margin and wider gap, but both fighters were persistent and brave, all the qualities their idol Hatton represented.
