British light heavyweight Buatsi earned a win yet unimpressive start to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a judges' verdict in a competitive ten-round fight at the Co-op Live Arena.
But Parker found success, landing some telling single shots in a match where the styles didn't complement each other, with excessive holding marring the fight.
Two judges judged it 96-94 to the winner, with the remaining official a even score.
"It wasn't my finest showing but what defines champions? They succeed," Buatsi commented in the ring as he was faced boos from the crowd.
The Londoner returned to winning ways after his first professional defeat - an thrilling contest with his previous opponent in February - to earn a 20th career victory.
Several boxing experts thought Parker, thirty-one, deserved the victory but the British boxer suffered the second loss of his 28-fight professional journey.
Buatsi Earns Victory in Slow Encounter
The fighter's advancement has plateaued in recent times through management issues and extended periods of inactivity.
Previously predicted as the rising talent of the British fight scene, he now is positioned trying to realize his potential - possibly in what feels like a final roll of the dice - under recent support.
He started carefully, with neither fighter willing to take risks in a cagey initial rounds.
The challenger, whose single earlier setback came at super middleweight against John Ryder in two years ago after damaging his hand during the bout, was making just his second fight at light heavyweight.
By the fight's midpoint, Parker had kept it close, creating difficulties for the favorite, who utilized a tight guard to barely take the more effective stanzas.
The crowd's patience diminished during constant clinching, with Parker falling to the floor multiple occasions during close exchanges.
The underdog ended stronger, but many rounds were hard to judge. Although, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just one round.
The victor advances, but with the presence of power puncher Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the journey to titles - on this evidence - still seems difficult.
Hatton Homage Fuels Local Rivalry Contest
In the supporting bout, Arthur beat Rea by divided decision in a excellent all-Manchester fight to capture the European light-heavyweight championship.
The arena grew hushed for a profoundly moving homage ahead of the contest as ten bells rang in memory of the city's native Ricky Hatton, who succumbed 46 years old in recently.
It was the first major televised match card in the location since Hatton's passing. As the final chime faded, the stillness became a shout of "only one Hatton".
The action during the bout reflected the feeling. Arthur was knocked down by a powerful punch squarely on the jaw in the second round, getting up unsteadily to his footing on the referee's count.
But the traditional red equipment of Arthur soon came forward again. From the other corner, Rea - in competitive blue - was damaged quickly, heavy swelling developing under his eye as the duo landed powerful shots.
Rea, twenty-seven, delivered effective strikes but Arthur, 34, fought fire with fire with effective uppercuts as the intensity hardly diminished over the full distance.
Arthur barely secured the decision with judges scorecards of draw, close margin and wider gap, but both fighters were determined and full of heart, everything their hero Hatton represented.
