UK light-heavyweight Buatsi secured a victory yet disappointing beginning to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a majority decision in a competitive 10-round fight at the boxing arena.
But Parker landed significant shots, delivering effective individual punches in a contest where the styles never quite clicked, with excessive holding disrupting the contest.
Two judges scored it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my greatest performance but how do champions respond? 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 maiden career setback - an epic battle with his previous opponent in February - to claim a 20th career victory.
Several boxing experts believed Parker, 31, deserved the victory but the contender experienced the second defeat of his 28-bout professional journey.
Buatsi Secures Victory in Uneventful Encounter
Buatsi's progress has slowed in recent times through promotional disputes and considerable time of inactivity.
Earlier forecast as the rising talent of British boxing, he now finds himself trying to realize his potential - possibly in what appears to be a final roll of the dice - under recent support.
He opened tentatively, with the two boxers ready to overcommit in a calculated initial rounds.
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 making just his second appearance at the higher weight class.
By the middle rounds, the underdog had stayed in contention, causing problems for the favorite, who worked behind a protective stance to narrowly win the more effective periods.
The crowd's patience diminished because of regular grabbing, with Parker slipping to the canvas various instances during grappling.
Parker ended stronger, but many rounds were challenging to assess. Nevertheless, ex-champion Carl Frampton - working with Dazn - had it particularly wide for the challenger, awarding Buatsi just only one stanza.
The victor advances, but with the caliber of power puncher Benavidez, and Russian star Bivol ruling the division, the path to championship success - on this evidence - still appears challenging.
Hatton Homage Motivates Manchester Derby Contest
In the co-main event, Arthur overcame Bradley Rea by divided decision in a excellent Manchester vs Manchester fight to win the continental belt.
The arena fell silent for a deeply poignant homage ahead of the contest as 10 bells rang in remembrance of the city's native Hatton, who died aged 46 in September.
It was the first major televised match card in the city since Hatton's death. As the concluding sound disappeared, the silence gave way to a chant of "there's just one Ricky Hatton".
The competition within the ring matched the sentiment. The eventual winner was knocked down by a left hook directly to the chin in the second round, getting up unsteadily to his feet on the count of eight.
But the traditional red shorts of the fighter soon advanced again. On the opposite side, the opponent - in rival City blue - was marked up quickly, heavy swelling developing under his orbital area as the fighters traded hard punches.
Rea, twenty-seven, delivered effective strikes but Arthur, thirty-four, matched the intensity with powerful upward punches as the action seldom decreased over the full distance.
Arthur narrowly won the result with judges scorecards of draw, close margin and 115-112, but the two athletes were persistent and full of heart, all the qualities their hero Hatton stood for.
