UK light-heavyweight Buatsi made a winning yet disappointing debut to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a majority decision in a close ten-round fight at the Manchester venue.
But Parker had his moments, connecting with powerful clean blows in a fight where the techniques failed to mesh, with frequent clinching disrupting the fight.
The scoring judges judged it 96-94 in favor to the winner, with the third a even score.
"It wasn't my best performance but how do champions respond? They prevail," Buatsi commented in the arena as he was greeted with negative reactions from the crowd.
The British fighter returned to winning ways after his maiden career setback - an thrilling contest with Callum Smith in February - to earn a twentieth professional win.
Several boxing experts felt Parker, thirty-one, merited the decision but the British boxer experienced the second defeat of his 28-fight career.
The Champion Secures Victory in Slow Encounter
The fighter's advancement has plateaued in recent years through management issues and extended periods of inactivity.
Earlier forecast as the next star of British boxing, he now is positioned trying to fulfil that early promise - perhaps in what seems like a final roll of the dice - under recent support.
He opened tentatively, with both competitors prepared to overcommit in a tentative initial rounds.
Parker, whose single earlier setback 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 bout at light-heavyweight.
By the middle rounds, Parker had stayed in contention, creating difficulties for the favorite, who utilized a protective stance to narrowly win the better rounds.
The audience's tolerance diminished amid regular grabbing, with the challenger falling to the floor several times during close exchanges.
Parker closed well, but multiple periods were difficult to score. Although, former world champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for Parker, scoring for Buatsi just only one stanza.
Buatsi advances, but with the presence of hard hitter Benavidez, and Eastern European fighter Bivol ruling the division, the road to world glory - judging by this showing - still seems difficult.
Hatton Memorial Fuels Local Rivalry Battle
In the secondary fight, Arthur beat Bradley Rea by split-decision in a outstanding Manchester vs Manchester contest to win the continental title.
The venue grew hushed for a profoundly moving homage before the fight as 10 bells tolled in remembrance of the city's native Hatton, who passed away 46 years old in last month.
It was the premier televised boxing event in the city since Hatton's death. As the final chime faded, the stillness transitioned to a chant of "there's just one Ricky Hatton".
The action during the bout equaled the feeling. Arthur was knocked down by a powerful punch flush on the chin in the early round, standing uncertainly to his position on the referee's count.
But the traditional red equipment of the fighter soon pressed ahead again. Across the ring, Rea - in opposing blue - was marked up quickly, heavy swelling developing under his orbital area as the pair traded powerful shots.
Rea, 27, connected with strong blows but the champion, thirty-four, responded in kind with effective uppercuts as the action hardly diminished over 12 rounds.
Arthur barely secured the verdict with judges scorecards of even score, narrow lead and wider gap, but both fighters were determined and brave, exactly what their idol Hatton stood for.
