British light-heavyweight Buatsi earned a win yet disappointing start to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output seemingly secured him a majority decision in a close 10-round bout at the Manchester venue.
But Parker landed significant shots, delivering effective individual punches in a fight where the approaches failed to mesh, with frequent clinching marring the action.
Two judges had it 96-94 to the winner, with the third a 95-95 draw.
"It wasn't my finest showing but how do champions respond? They succeed," Buatsi stated in the ring as he was met by negative reactions from the spectators.
The British fighter got back to winning after his initial pro loss - an thrilling contest with his previous opponent in February - to secure a 20th pro success.
Various analysts felt the challenger, 31, deserved the victory but the Derbyshire fighter suffered the second defeat of his 28-match career.
Buatsi Earns Victory in Uneventful Match
Buatsi's progress has slowed in recent times through promotional disputes and long spells of lack of fights.
Previously predicted as the next star of British boxing, he now finds himself trying to fulfil that early promise - possibly in what seems like a ultimate opportunity - under fresh management.
He began cautiously, with neither fighter willing to take risks in a calculated initial rounds.
The challenger, whose sole prior loss came at super middleweight against John Ryder in 2022 after breaking his hand during the bout, was making just his second bout at the higher weight class.
By the halfway stage, Parker had stayed in contention, causing problems for the favorite, who utilized a tight guard to edge the better periods.
The audience's tolerance decreased amid regular grabbing, with the challenger falling to the floor various instances during close exchanges.
The underdog ended stronger, but many rounds were challenging to assess. Nevertheless, former world champion the boxing analyst - working with Dazn - had it significantly in favor for Parker, giving Buatsi just one round.
Buatsi progresses, but with the presence of heavy hitter Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - based on this performance - still looks steep.
Hatton Memorial Motivates Manchester Derby Fight
In the secondary fight, Arthur defeated Bradley Rea by divided decision in a outstanding Manchester vs Manchester fight to win the European light-heavyweight championship.
The arena fell silent for a extremely emotional tribute ahead of the contest as a bell ceremony rang in honor of Manchester's own Ricky Hatton, who succumbed aged 46 in recently.
It was the premier televised match card in the location since the passing of Hatton. As the final chime faded, the quiet transitioned to a shout of "there's only one Ricky Hatton".
The fighting within the ring equaled the feeling. The eventual winner was knocked down by a powerful punch squarely on the face in the early round, rising shakily to his feet on the count of eight.
But the traditional red attire of Arthur soon came forward again. From the other corner, the opponent - in rival City blue - was damaged early, significant bruising forming under his face as the fighters exchanged punishing blows.
Rea, 27, landed power punches but Arthur, thirty-four, responded in kind with powerful upward punches as the intensity rarely dropped over twelve rounds.
Arthur narrowly won the result with judges scorecards of even score, narrow lead and 115-112, but the two athletes were determined and full of heart, all the qualities their inspiration Hatton stood for.
