UK light-heavyweight Buatsi secured a victory yet unimpressive start to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a split decision in a close 10-round bout at the Co-op Live Arena.
But Parker found success, connecting with powerful single shots in a contest where the styles failed to mesh, with excessive holding marring the action.
The scoring judges judged it 96-94 in favor to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest showing but what do champions do? They prevail," the victor commented in the arena as he was met by negative reactions from the audience.
The British fighter returned to winning ways after his first professional defeat - an thrilling contest with Callum Smith in February - to earn a twentieth professional win.
Various analysts believed Parker, 31, deserved the victory but the contender suffered the second setback of his 28-match professional journey.
The Champion Claims Win in Slow Match
His career development has stalled in the past few years through contract problems and long spells of inactivity.
Earlier forecast as the next star of British boxing, he now stands trying to realize his potential - maybe in what seems like a ultimate opportunity - under recent support.
He started carefully, with both competitors prepared to be aggressive in a cagey early stages.
Parker, whose single earlier setback came at the lower weight class against John Ryder in two years ago after suffering a hand injury in the contest, was participating in his second bout at light heavyweight.
By the halfway stage, the underdog had stayed in contention, causing problems for Buatsi, who operated with a tight guard to barely take the cleaner rounds.
The spectators' acceptance wore thin during frequent holding, with Parker falling to the floor various instances during grappling.
The underdog closed well, but multiple periods were challenging to assess. Nevertheless, former world champion Carl Frampton - working with Dazn - had it significantly in favor for the challenger, scoring for Buatsi just a single round.
The victor advances, but with the presence of hard hitter David Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Tribute Motivates Manchester Derby Contest
In the co-main event, Lyndon Arthur beat Bradley Rea by split-decision in a excellent all-Manchester fight to win the continental belt.
The arena became quiet for a extremely emotional homage before the fight as ten bells sounded in remembrance of Manchester's own Hatton, who passed away at age 46 in September.
It was the premier televised match card in the urban center since the passing of Hatton. As the final chime faded, the quiet became a roar of "only one Hatton".
The action inside the ropes matched the feeling. Arthur was knocked down by a powerful punch squarely on the face in the early round, standing uncertainly to his feet on the eight count.
But the red-colored attire of the fighter soon came forward again. From the other corner, the opponent - in rival City blue - was damaged soon, heavy swelling developing under his face as the fighters traded hard punches.
Rea, 27, delivered effective strikes but the champion, thirty-four, matched the intensity with powerful upward punches as the pace seldom decreased over the full distance.
Arthur barely secured the decision with official scores of draw, close margin and wider gap, but both fighters were determined and brave, everything their idol Hatton embodied.
