British light-heavyweight Buatsi secured a victory yet unimpressive start to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a split decision in a competitive 10-round contest at the Co-op Live Arena.
But the challenger had his moments, landing some telling single shots in a match where the approaches failed to mesh, with frequent clinching marring the contest.
The scoring judges had it 96-94 for to Buatsi, with the third a even score.
"It wasn't my finest showing but what defines champions? They win," the victor said in the arena as he was faced disapproval from the audience.
The British fighter returned to winning ways after his first professional defeat - an memorable fight with his previous opponent in last winter - to earn a 20th career victory.
Various analysts believed Parker, thirty-one, merited the decision but the contender endured the second defeat of his 28-match career.
The Champion Secures Victory in Slow Affair
Buatsi's progress has stalled in the past few years through contract problems and extended periods of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now is positioned trying to achieve expectations - perhaps in what appears to be a ultimate opportunity - under fresh management.
He began cautiously, with both competitors willing to be aggressive in a cagey opening few rounds.
Parker, whose single earlier setback came at super middleweight against John Ryder in two years ago after suffering a hand injury during the bout, was participating in his second appearance at the higher weight class.
By the middle rounds, the underdog had stayed in contention, making life awkward for Buatsi, who operated with a tight guard to edge the better periods.
The spectators' acceptance wore thin amid regular grabbing, with the challenger slipping to the canvas multiple occasions during clinches.
Parker ended stronger, but several stanzas were difficult to score. Nevertheless, former world champion Carl Frampton - working with Dazn - had it particularly wide for the challenger, giving Buatsi just a single round.
Buatsi advances, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - on this evidence - still appears challenging.
Hatton Memorial Motivates All-Manchester Battle
In the co-main event, Arthur defeated Rea by close verdict in a excellent Manchester vs Manchester battle to win the European light heavyweight title.
The venue grew hushed for a deeply poignant memorial before the fight as ten bells sounded in honor of local hero Hatton, who died at age 46 in last month.
It was the first major televised fight night in the urban center since Hatton's death. As the final chime ended, the silence gave way to a chant of "there's only one Hatton".
The competition during the bout reflected the feeling. Arthur was floored by a significant blow flush on the jaw in the initial stage, standing uncertainly to his footing on the referee's count.
But the traditional red shorts of the fighter soon advanced again. On the opposite side, the opponent - in rival City blue - was marked up early, significant bruising forming under his face as the duo exchanged hard punches.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, fought fire with fire with powerful upward punches as the pace rarely dropped over 12 rounds.
The victor barely secured the verdict with official scores of draw, 115-113 and wider gap, but each competitor were relentless and full of heart, everything their idol Hatton embodied.
