British light-heavyweight Buatsi made a winning yet underwhelming beginning to life under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a split decision in a tight 10-round contest at the boxing arena.
But Parker had his moments, connecting with powerful clean blows in a contest where the techniques never quite clicked, with frequent clinching marring the action.
The scoring judges scored it 96-94 for to the winner, with the third a 95-95.
"It wasn't my greatest performance but what do champions do? They succeed," the victor said in the arena as he was faced boos from the crowd.
The Londoner bounced back after his initial pro loss - an memorable fight with his previous opponent in February - to secure a twentieth professional win.
Several boxing experts believed the challenger, thirty-one, should have been awarded the win but the Derbyshire fighter endured the second loss of his 28-bout career.
The Champion Earns Victory in Uneventful Affair
The fighter's advancement has stalled in recent years through contract problems and extended periods of lack of fights.
Once tipped as the next star of the British fight scene, he now is positioned trying to achieve expectations - possibly in what seems like a last chance - under new promotional backing.
He opened tentatively, with neither fighter ready to overcommit in a calculated initial rounds.
Parker, whose sole prior loss came at the lower weight class against John Ryder in 2022 after breaking his hand in the contest, was making just his second bout at the higher weight class.
By the fight's midpoint, the underdog had kept it close, making life awkward for Buatsi, who worked behind a protective stance to barely take the more effective stanzas.
The crowd's patience diminished because of regular grabbing, with Parker going down various instances during clinches.
Parker closed well, but multiple periods were difficult to score. Although, ex-champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for Parker, giving Buatsi just only one stanza.
The victor moves on, but with the caliber of hard hitter Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Tribute Inspires All-Manchester Contest
In the co-main event, Arthur beat Bradley Rea by divided decision in a terrific local rivalry battle to win the continental title.
The stadium fell silent for a extremely emotional homage ahead of the contest as ten bells tolled in memory of the city's native Hatton, who succumbed aged 46 in last month.
It was the premier televised match card in the city since Hatton's death. As the final chime ended, the silence transitioned to a chant of "there's only one Hatton".
The action during the bout equaled the feeling. The eventual winner was sent to the canvas by a significant blow directly to the jaw in the initial stage, standing uncertainly to his position on the eight count.
But the traditional red shorts of the fighter soon advanced again. Across the ring, Rea - in rival City blue - was injured soon, significant bruising appearing under his face as the fighters landed punishing blows.
The challenger, 27, landed power punches but the champion, thirty-four, responded in kind with stinging uppercuts as the pace rarely dropped over the full distance.
Arthur narrowly won the verdict with official scores of 114-114, close margin and clearer margin, but the two athletes were persistent and full of heart, all the qualities their inspiration the legend represented.
