UK light-heavyweight Buatsi secured a victory yet disappointing start to life under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a close 10-round bout at the Manchester venue.
But the challenger found success, delivering effective clean blows in a contest where the techniques didn't complement each other, with excessive holding disrupting the action.
The scoring judges scored it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my greatest showing but what do champions do? They win," Buatsi commented in the ringside as he was greeted with negative reactions from the crowd.
The British fighter got back to winning after his first professional defeat - an memorable fight with Callum Smith in February - to secure a twentieth professional win.
Various analysts felt the challenger, 31, should have been awarded the win but the British boxer experienced the second defeat of his 28-match professional journey.
The Champion Claims Triumph in Dull Match
His career development has slowed in recent years through contract problems and extended periods of lack of fights.
Once tipped as the future of UK boxing, he now is positioned trying to fulfil that early promise - possibly in what appears to be a final roll of the dice - under new promotional backing.
He opened tentatively, with both competitors prepared to be aggressive in a cagey opening few rounds.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in 2022 after damaging his hand in the contest, was competing in only his second fight at light heavyweight.
By the halfway stage, Parker had kept it close, causing problems for Buatsi, who operated with a solid defense to barely take the cleaner stanzas.
The audience's tolerance wore thin during regular grabbing, with the challenger falling to the floor various instances during grappling.
Parker finished the stronger, but several stanzas were hard to judge. However, ex-champion Carl Frampton - commentating on Dazn - had it significantly in favor for Parker, giving Buatsi just a single round.
Buatsi progresses, but with the presence of hard hitter Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - based on this performance - still looks steep.
Hatton Memorial Motivates Manchester Derby Fight
In the co-main event, Lyndon Arthur defeated Bradley Rea by close verdict in a excellent all-Manchester fight to secure the European light heavyweight championship.
The stadium fell silent for a deeply poignant tribute before the fight as a bell ceremony tolled in honor of the city's native Ricky Hatton, who succumbed aged 46 in last month.
It was the initial significant broadcast match card in the location since Hatton's death. As the concluding sound disappeared, the quiet transitioned to a roar of "there's just one Ricky Hatton".
The competition during the bout matched the feeling. The eventual winner was knocked down by a powerful punch directly to the jaw in the second round, standing uncertainly to his position on the referee's count.
But the red-colored shorts of the fighter soon pressed ahead again. On the opposite side, the opponent - in opposing blue - was marked up quickly, noticeable damage developing under his orbital area as the fighters traded hard punches.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, fought fire with fire with powerful upward punches as the pace hardly diminished over twelve rounds.
Arthur narrowly won the result with scoring tallies of even score, 115-113 and wider gap, but each competitor were determined and full of heart, exactly what their idol the legend embodied.
