UK light heavyweight Buatsi secured a victory yet underwhelming beginning to his career under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's work rate seemingly secured him a judges' verdict in a close ten-round bout at the Co-op Live Arena.
But Parker landed significant shots, landing some telling single shots in a match where the styles didn't complement each other, with excessive holding marring the action.
The scoring judges judged it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my best display but what defines champions? They succeed," the victor said in the ringside as he was greeted with disapproval from the spectators.
The Londoner bounced back after his initial pro loss - an memorable fight with Callum Smith in February - to earn a twentieth professional win.
Some ringside observers felt Parker, 31, should have been awarded the win but the Derbyshire fighter suffered the second defeat of his 28-match career.
Buatsi Secures Win in Slow Affair
Buatsi's progress has slowed in recent times through promotional disputes and extended periods of lack of fights.
Earlier forecast as the rising talent of the British fight scene, he now stands trying to fulfil that early promise - perhaps in what seems like a ultimate opportunity - under new promotional backing.
He opened tentatively, with the two boxers ready to take risks in a calculated opening few rounds.
Parker, whose sole prior loss came at the lower weight class against his former opponent in 2022 after suffering a hand injury mid-fight, was competing in only his second appearance at light-heavyweight.
By the fight's midpoint, Parker had made it competitive, making life awkward for Buatsi, who operated with a tight guard to edge the better rounds.
The spectators' acceptance wore thin during frequent holding, with the challenger falling to the floor various instances during close exchanges.
Parker closed well, but several stanzas were challenging to assess. Although, previous titleholder Carl Frampton - providing analysis for Dazn - had it significantly in favor for Parker, giving Buatsi just one round.
Buatsi moves on, but with the presence of power puncher Benavidez, and Eastern European fighter Bivol dominating the weight class, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Homage Fuels Local Rivalry Battle
In the co-main event, Arthur beat Bradley Rea by close verdict in a outstanding all-Manchester fight to win the continental title.
The venue fell silent for a extremely emotional memorial prior to the bout as 10 bells rang in remembrance of Manchester's own Hatton, who died at age 46 in September.
It was the premier televised fight night in the urban center since the passing of Hatton. As the final chime disappeared, the stillness became a roar of "there's only one Hatton".
The action inside the ropes matched the feeling. The eventual winner was floored by a significant blow directly to the face in the initial stage, getting up unsteadily to his feet on the eight count.
But the traditional red shorts of the fighter soon advanced again. Across the ring, the opponent - in competitive blue - was marked up soon, noticeable damage appearing under his face as the duo landed punishing blows.
The challenger, twenty-seven, connected with strong blows but the champion, 34, fought fire with fire with stinging uppercuts as the action rarely dropped over 12 rounds.
The victor narrowly won the verdict with scoring tallies of even score, 115-113 and clearer margin, but each competitor were determined and courageous, all the qualities their idol Hatton represented.
