British 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 work rate seemingly secured him a majority decision in a tight 10-round bout at the Manchester venue.
But Parker landed significant shots, connecting with powerful clean blows in a match where the approaches never quite clicked, with constant grabbing spoiling the action.
The scoring judges had it 96-94 for to Buatsi, with the third a even score.
"It wasn't my finest performance but what defines champions? They succeed," Buatsi commented in the ring as he was greeted with disapproval from the audience.
The Londoner bounced back after his initial pro loss - an epic battle with Callum Smith in last winter - to claim a 20th pro success.
Some ringside observers felt Parker, 31, merited the decision but the British boxer experienced the second loss of his 28-bout career.
The Champion Claims Triumph in Slow Match
The fighter's advancement has plateaued in recent years through contract problems and extended periods of inactivity.
Previously predicted as the future of UK boxing, he now stands trying to fulfil that early promise - perhaps in what feels like a ultimate opportunity - under recent support.
He began cautiously, with neither fighter ready to overcommit in a tentative initial rounds.
The challenger, whose sole prior loss came at the lower weight class against John Ryder in 2022 after breaking his hand in the contest, was competing in only his second fight at light heavyweight.
By the fight's midpoint, the underdog had stayed in contention, making life awkward for the favorite, who utilized a protective stance to narrowly win the better stanzas.
The crowd's patience wore thin because of constant clinching, with Parker going down various instances during close exchanges.
Parker ended stronger, but many rounds were challenging to assess. Nevertheless, ex-champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for the challenger, scoring for Buatsi just a single round.
Buatsi progresses, but with the presence of heavy hitter Benavidez, and Russian star Bivol dominating the weight class, the path to championship success - based on this performance - still seems difficult.
Ricky Hatton Memorial Motivates All-Manchester Battle
In the co-main event, Arthur beat Rea by divided decision in a terrific local rivalry fight to secure the European light-heavyweight belt.
The venue fell silent for a extremely emotional memorial ahead of the contest as 10 bells rang in remembrance of the city's native Hatton, who passed away at age 46 in September.
It was the initial significant broadcast boxing event in the city since Hatton's passing. As the final chime faded, the quiet transitioned to a shout of "only one Hatton".
The action during the bout reflected the feeling. Arthur was knocked down by a left hook squarely on the face in the second round, rising shakily to his feet on the referee's count.
But the red-colored equipment of Arthur soon came forward again. From the other corner, Rea - in competitive blue - was injured soon, significant bruising forming under his orbital area as the fighters traded punishing blows.
The challenger, 27, connected with strong blows but Arthur, 34, fought fire with fire with powerful upward punches as the pace hardly diminished over the full distance.
The victor narrowly won the result with judges scorecards of draw, 115-113 and wider gap, but both fighters were determined and brave, exactly what their hero the legend represented.
