UK light heavyweight Buatsi made a winning yet unimpressive debut to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate seemingly secured him a split decision in a close ten-round fight at the Co-op Live Arena.
But Parker landed significant shots, delivering effective single shots in a fight where the techniques never quite clicked, with excessive holding spoiling the action.
The officials scored it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my greatest showing but what do champions do? They win," the victor stated in the ringside as he was met by boos from the crowd.
The Londoner got back to winning after his maiden career setback - an thrilling contest with Callum Smith in last winter - to claim a 20th pro success.
Some ringside observers thought the challenger, 31, merited the victory but the British boxer experienced the second defeat of his 28-match professional journey.
The Champion Secures Victory in Uneventful Match
The fighter's advancement has slowed in recent years through contract problems and considerable time of lack of fights.
Once tipped as the next star of the British fight scene, he now stands trying to achieve expectations - possibly in what feels like a ultimate opportunity - under new promotional backing.
He began cautiously, with neither fighter ready to take risks in a cagey early stages.
Parker, whose only previous defeat came at the lower weight class against John Ryder in two years ago after damaging his hand in the contest, was competing in only his second bout at light-heavyweight.
By the fight's midpoint, Parker had made it competitive, creating difficulties for the favorite, who worked behind a solid defense to barely take the better rounds.
The audience's tolerance decreased amid frequent holding, with Parker slipping to the canvas various instances during close exchanges.
The underdog finished the stronger, but multiple periods were difficult to score. However, ex-champion the boxing analyst - working with the broadcast network - had it particularly wide for the challenger, giving Buatsi just one round.
The victor advances, but with the likes of heavy hitter David Benavidez, and Russian champion Bivol ruling the division, the path to championship success - on this evidence - still looks steep.
Ricky Hatton Memorial Inspires All-Manchester Battle
In the supporting bout, Arthur defeated Rea by split-decision in a terrific Manchester vs Manchester contest to win the European light heavyweight belt.
The arena became quiet for a deeply poignant memorial ahead of the contest as a bell ceremony tolled in honor of Manchester's own Ricky Hatton, who died 46 years old in last month.
It was the premier televised match card in the location since Hatton's death. As the concluding sound faded, the quiet became a roar of "there's only one Hatton".
The competition within the ring equaled the feeling. Arthur was floored by a left hook squarely on the chin in the early round, standing uncertainly to his feet on the eight count.
But the traditional red shorts of the fighter soon advanced again. From the other corner, the opponent - in opposing blue - was injured quickly, noticeable damage appearing under his face as the pair landed punishing blows.
Rea, twenty-seven, landed power punches but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the action hardly diminished over twelve rounds.
Arthur narrowly won the result with judges scorecards of 114-114, narrow lead and wider gap, but the two athletes were relentless and courageous, all the qualities their hero Hatton represented.
