British light heavyweight Buatsi secured a victory yet unimpressive debut to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a split decision in a tight ten-round fight at the Manchester venue.
But the challenger had his moments, landing some telling individual punches in a fight where the techniques failed to mesh, with constant grabbing disrupting the fight.
The scoring judges judged it 96-94 for to the winner, with the final judge a 95-95.
"It wasn't my best showing but what do champions do? They succeed," the victor said in the ring as he was faced negative reactions from the crowd.
The Londoner bounced back after his initial pro loss - an epic battle with Callum Smith in February - to earn a 20th pro success.
Several boxing experts felt Parker, thirty-one, deserved the decision but the contender endured the second loss of his 28-bout professional journey.
The Champion Claims Victory in Uneventful Encounter
The fighter's advancement has slowed in recent times through management issues and long spells of inactivity.
Once tipped as the next star of the British fight scene, he now finds himself trying to achieve expectations - maybe in what seems like a ultimate opportunity - under fresh management.
He began cautiously, with the two boxers prepared to be aggressive in a calculated opening few rounds.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in 2022 after breaking his hand mid-fight, was competing in only his second fight at light heavyweight.
By the fight's midpoint, the underdog had made it competitive, causing problems for the favorite, who worked behind a protective stance to edge the more effective stanzas.
The crowd's patience decreased because of constant clinching, with Parker going down several times during clinches.
Parker closed well, but multiple periods were hard to judge. Although, former world champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for Parker, awarding Buatsi just a single round.
The victor progresses, but with the presence of heavy hitter Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - on this evidence - still appears challenging.
Hatton Homage Fuels All-Manchester Fight
In the secondary fight, Arthur overcame Bradley Rea by divided decision in a terrific all-Manchester fight to secure the European light-heavyweight title.
The arena grew hushed for a extremely emotional homage prior to the bout as ten bells tolled in remembrance of Manchester's own Ricky Hatton, who passed away at age 46 in last month.
It was the premier televised match card in the city since the passing of Hatton. As the final chime disappeared, the stillness became a chant of "only one Hatton".
The fighting inside the ropes matched the emotion. The eventual winner was knocked down by a left hook flush on the jaw in the early round, standing uncertainly to his footing on the count of eight.
But the Manchester United-red attire of the fighter soon pressed ahead again. From the other corner, Rea - in competitive blue - was marked up quickly, heavy swelling appearing under his face as the pair exchanged 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 decision with official scores of draw, 115-113 and 115-112, but both fighters were persistent and brave, all the qualities their idol the legend represented.
