UK light heavyweight Buatsi earned a win yet underwhelming beginning to his career under new promoters with a unanimous decision over Zach Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a judges' verdict in a competitive 10-round bout at the Co-op Live Arena.
But Parker found success, delivering effective clean blows in a fight where the approaches failed to mesh, with frequent clinching disrupting the contest.
The officials had it 96-94 in favor to Buatsi, with the third a 95-95 draw.
"It wasn't my best performance but what defines champions? They succeed," Buatsi stated in the ring as he was met by disapproval from the crowd.
The British fighter bounced back after his first professional defeat - an memorable fight with his previous opponent in February - to secure a 20th career victory.
Some ringside observers felt the challenger, thirty-one, deserved the victory but the Derbyshire fighter suffered the second defeat of his 28-match professional journey.
The Champion Earns Victory in Dull Affair
Buatsi's progress has plateaued in recent years through management issues and extended periods of inactivity.
Once tipped as the rising talent of the British fight scene, he now is positioned trying to fulfil that early promise - maybe in what seems like a final roll of the dice - under recent support.
He opened tentatively, with neither fighter willing to overcommit in a calculated early stages.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in two years ago after suffering a hand injury in the contest, was participating in his second bout at light heavyweight.
By the fight's midpoint, Parker had stayed in contention, causing problems for Buatsi, who worked behind a solid defense to narrowly win the more effective rounds.
The audience's tolerance decreased amid regular grabbing, with the challenger falling to the floor various instances during clinches.
The underdog finished the stronger, but multiple periods were difficult to score. However, previous titleholder Carl Frampton - commentating on the broadcast network - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
The victor advances, but with the caliber of power puncher Benavidez, and Russian star Bivol ruling the division, the journey to titles - based on this performance - still seems difficult.
Ricky Hatton Memorial Fuels Manchester Derby Contest
In the secondary fight, Lyndon Arthur overcame Rea by split-decision in a excellent Manchester vs Manchester battle to secure the continental championship.
The stadium fell silent for a profoundly moving homage prior to the bout as a bell ceremony rang in remembrance of local hero Hatton, who succumbed aged 46 in last month.
It was the premier televised boxing event in the city since Hatton's passing. As the last bell disappeared, the silence gave way to a chant of "there's only one Ricky Hatton".
The competition inside the ropes matched the feeling. The eventual winner was floored by a significant blow directly to the face in the initial stage, getting up unsteadily to his footing on the referee's count.
But the red-colored attire of Arthur soon came forward again. On the opposite side, Rea - in competitive blue - was marked up early, noticeable damage appearing under his orbital area as the fighters landed powerful shots.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, matched the intensity with powerful upward punches as the intensity hardly diminished over the full distance.
Arthur narrowly won the verdict with judges scorecards of 114-114, close margin and wider gap, but the two athletes were determined and full of heart, everything their idol the legend represented.
