British light-heavyweight Buatsi earned a win yet underwhelming beginning to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a judges' verdict in a close 10-round contest at the Manchester venue.
But Parker landed significant shots, landing some telling clean blows in a contest where the approaches failed to mesh, with frequent clinching disrupting the contest.
The officials had it 96-94 for to Buatsi, with the final judge a even score.
"It wasn't my best showing but what defines champions? They succeed," Buatsi said in the ring as he was faced boos from the crowd.
The British fighter got back to winning after his initial pro loss - an thrilling contest with Callum Smith in last winter - to earn a twentieth professional win.
Several boxing experts thought Parker, 31, deserved the decision but the British boxer suffered the second loss of his 28-fight professional journey.
The Champion Claims Victory in Uneventful Affair
His career development has plateaued in the past few years through promotional disputes and extended periods of lack of fights.
Earlier forecast as the next star of British boxing, he now finds himself trying to realize his potential - maybe in what seems like a final roll of the dice - under recent support.
He started carefully, with neither fighter willing to take risks in a tentative initial rounds.
Parker, whose only previous defeat came at super middleweight against John Ryder in two years ago after damaging his hand mid-fight, was participating in his second appearance at light heavyweight.
By the halfway stage, Parker had kept it close, making life awkward for Buatsi, who worked behind a solid defense to edge the cleaner stanzas.
The crowd's patience diminished during regular grabbing, with Parker slipping to the canvas various instances during close exchanges.
Parker ended stronger, but several stanzas were difficult to score. However, ex-champion Carl Frampton - providing analysis for Dazn - had it strongly leaning for Parker, scoring for Buatsi just a single round.
Buatsi advances, but with the presence of power puncher Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still appears challenging.
Hatton Memorial Inspires All-Manchester Battle
In the co-main event, Arthur overcame Rea by close verdict in a excellent Manchester vs Manchester contest to secure the European light-heavyweight championship.
The venue fell silent for a profoundly moving homage prior to the bout as ten bells sounded in memory of local hero Hatton, who passed away 46 years old in last month.
It was the premier televised fight night in the city since Hatton's passing. As the final chime faded, the quiet gave way to a chant of "there's just one Ricky Hatton".
The fighting during the bout reflected the sentiment. The eventual winner was knocked down by a significant blow flush on the jaw in the second round, standing uncertainly to his position on the referee's count.
But the Manchester United-red attire of Arthur soon pressed ahead again. On the opposite side, Rea - in opposing blue - was marked up quickly, heavy swelling developing under his face as the fighters exchanged hard punches.
Rea, 27, delivered effective strikes but Arthur, thirty-four, fought fire with fire with effective uppercuts as the pace hardly diminished over 12 rounds.
Arthur barely secured the decision with scoring tallies of even score, close margin and wider gap, but the two athletes were relentless and full of heart, everything their inspiration the legend embodied.
