British light-heavyweight Buatsi earned a win yet unimpressive debut to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a majority decision in a competitive 10-round fight at the Manchester venue.
But Parker had his moments, connecting with powerful individual punches in a contest where the techniques failed to mesh, with excessive holding spoiling the action.
The scoring judges had it 96-94 for to the winner, with the third a 95-95.
"It wasn't my greatest display but how do champions respond? They succeed," the victor said in the ringside as he was faced boos from the spectators.
The British fighter bounced back after his first professional defeat - an epic battle with his previous opponent in February - to secure a 20th pro success.
Some ringside observers felt Parker, 31, merited the victory but the contender suffered the second setback of his 28-bout professional journey.
The Champion Secures Triumph in Slow Match
Buatsi's progress has plateaued in recent times through management issues and extended periods of inactivity.
Earlier forecast as the next star of the British fight scene, he now stands trying to fulfil that early promise - perhaps in what feels like a last chance - under fresh management.
He started carefully, with both competitors willing to take risks in a cagey opening few rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in 2022 after damaging his hand mid-fight, was competing in only his second appearance at the higher weight class.
By the middle rounds, Parker had stayed in contention, causing problems for the favorite, who utilized a protective stance to edge the cleaner stanzas.
The audience's tolerance decreased during regular grabbing, with the challenger falling to the floor multiple occasions during close exchanges.
The underdog finished the stronger, but many rounds were hard to judge. Although, ex-champion Carl Frampton - providing analysis for Dazn - had it particularly wide for the challenger, giving Buatsi just a single round.
Buatsi advances, but with the likes of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the journey to titles - based on this performance - still looks steep.
Hatton Tribute Inspires All-Manchester Contest
In the secondary fight, Lyndon Arthur overcame Rea by close verdict in a excellent all-Manchester contest to secure the continental belt.
The arena fell silent for a deeply poignant homage ahead of the contest as a bell ceremony rang in honor of the city's native Ricky Hatton, who succumbed aged 46 in last month.
It was the premier televised fight night in the location since Hatton's passing. As the last bell ended, the quiet transitioned to a shout of "there's only one Ricky Hatton".
The competition during the bout matched the sentiment. The eventual winner was knocked down by a significant blow directly to the jaw in the early round, getting up unsteadily to his position on the referee's count.
But the red-colored attire of Arthur soon advanced again. From the other corner, Rea - in rival City blue - was marked up quickly, heavy swelling appearing under his face as the pair traded powerful shots.
Rea, 27, landed power punches but the champion, 34, responded in kind with powerful upward punches as the action rarely dropped over twelve rounds.
The victor narrowly won the verdict with scoring tallies of 114-114, close margin and wider gap, but each competitor were determined and full of heart, everything their hero Hatton stood for.
