British light-heavyweight Buatsi made a winning yet underwhelming beginning to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a tight ten-round fight at the boxing arena.
But Parker found success, connecting with powerful clean blows in a fight where the approaches never quite clicked, with frequent clinching spoiling the fight.
The officials scored it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my finest display but how do champions respond? They prevail," the victor stated in the ringside as he was greeted with negative reactions from the crowd.
The Londoner bounced back after his first professional defeat - an thrilling contest with Callum Smith in February - to claim a twentieth professional win.
Several boxing experts felt Parker, 31, merited the decision but the contender endured the second defeat of his 28-match professional journey.
Buatsi Secures Victory in Uneventful Affair
Buatsi's progress has slowed in recent years through promotional disputes and long spells of lack of fights.
Previously predicted as the rising talent of UK boxing, he now finds himself trying to realize his potential - perhaps in what seems like a last chance - under recent support.
He opened tentatively, with neither fighter prepared to take risks in a tentative opening few rounds.
Parker, whose single earlier setback came at super middleweight against his former opponent in two years ago after suffering a hand injury mid-fight, was competing in only his second bout at light heavyweight.
By the middle rounds, the underdog had made it competitive, creating difficulties for the favorite, who operated with a solid defense to edge the cleaner rounds.
The audience's tolerance decreased amid frequent holding, with the challenger going down several times during grappling.
The underdog ended stronger, but multiple periods were hard to judge. Nevertheless, previous titleholder Carl Frampton - working with the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just a single round.
Buatsi advances, but with the presence of hard hitter Benavidez, and Russian champion Bivol ruling the division, the journey to titles - judging by this showing - still appears challenging.
Hatton Tribute Fuels All-Manchester Contest
In the supporting bout, Arthur beat Rea by close verdict in a outstanding local rivalry battle to secure the European light-heavyweight belt.
The stadium grew hushed for a profoundly moving homage ahead of the contest as ten bells sounded in honor of local hero Hatton, who passed away 46 years old in September.
It was the first major televised fight night in the city since Hatton's death. As the concluding sound ended, the quiet became a roar of "only one Hatton".
The action during the bout equaled the emotion. Arthur was knocked down by a powerful punch flush on the face in the second round, rising shakily to his feet on the referee's count.
But the Manchester United-red attire of the fighter soon pressed ahead again. On the opposite side, Rea - in opposing blue - was marked up soon, heavy swelling forming under his orbital area as the duo traded punishing blows.
The challenger, twenty-seven, delivered effective strikes but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the pace hardly diminished over the full distance.
Arthur edged the result with judges scorecards of 114-114, 115-113 and clearer margin, but the two athletes were persistent and full of heart, all the qualities their idol the legend represented.
