British light-heavyweight Buatsi secured a victory yet disappointing debut to his career under recent management 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 tight 10-round fight at the Co-op Live Arena.
But Parker found success, connecting with powerful single shots in a fight where the styles failed to mesh, with excessive holding spoiling the fight.
Two judges had it 96-94 for to Buatsi, with the remaining official a 95-95.
"It wasn't my finest performance but what defines champions? They succeed," the victor stated in the ring as he was greeted with disapproval from the crowd.
The Londoner bounced back after his initial pro loss - an epic battle with his previous opponent in last winter - to secure a twentieth professional win.
Several boxing experts thought Parker, thirty-one, should have been awarded the decision but the British boxer suffered the second setback of his 28-fight career.
Buatsi Earns Triumph in Uneventful Affair
The fighter's advancement has slowed in recent times through contract problems and extended periods of inactivity.
Earlier forecast as the rising talent of British boxing, he now is positioned trying to realize his potential - possibly in what seems like a ultimate opportunity - under fresh management.
He started carefully, with both competitors ready to take risks in a cagey opening few rounds.
Parker, whose only previous defeat came at super-middleweight against his former opponent in 2022 after breaking his hand mid-fight, was competing in only his second bout at the higher weight class.
By the middle rounds, Parker had stayed in contention, making life awkward for the favorite, who worked behind a tight guard to barely take the better periods.
The spectators' acceptance decreased because of regular grabbing, with Parker slipping to the canvas several times during clinches.
Parker ended stronger, but several stanzas were challenging to assess. However, former world champion the boxing analyst - commentating on Dazn - had it particularly wide for the challenger, giving Buatsi just one round.
Buatsi moves on, but with the presence of heavy hitter Benavidez, and Russian champion Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Homage Fuels Local Rivalry Fight
In the supporting bout, Arthur defeated Rea by divided decision in a excellent Manchester vs Manchester battle to capture the European light heavyweight belt.
The venue grew hushed for a extremely emotional tribute before the fight as 10 bells sounded in remembrance of local hero Hatton, who passed away at age 46 in September.
It was the premier televised fight night in the location since Hatton's death. As the last bell ended, the silence gave way to a shout of "there's just one Ricky Hatton".
The competition within the ring matched the sentiment. Arthur was sent to the canvas by a significant blow flush on the chin in the initial stage, standing uncertainly to his feet on the eight count.
But the Manchester United-red attire of the fighter soon advanced again. Across the ring, the opponent - in rival City blue - was injured soon, noticeable damage appearing under his eye as the fighters landed punishing blows.
The challenger, twenty-seven, connected with strong blows but the champion, 34, responded in kind with powerful upward punches as the pace seldom decreased over twelve rounds.
The victor narrowly won the result with scoring tallies of draw, close margin and clearer margin, but both fighters were determined and courageous, everything their inspiration Hatton embodied.
