British light heavyweight Buatsi earned a win yet disappointing debut to his career under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a judges' verdict in a tight ten-round contest at the Co-op Live Arena.
But the challenger had his moments, landing some telling clean blows in a fight where the styles didn't complement each other, with constant grabbing spoiling the contest.
The officials scored it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest showing but what defines champions? They succeed," Buatsi commented in the ring as he was met by negative reactions from the audience.
The British fighter got back to winning after his first professional defeat - an thrilling contest with Callum Smith in last winter - to secure a 20th career victory.
Several boxing experts thought Parker, thirty-one, should have been awarded the decision but the contender experienced the second loss of his 28-match career.
The Champion Claims Triumph in Uneventful Match
The fighter's advancement has stalled in recent times through contract problems and considerable time of lack of fights.
Previously predicted as the future of UK boxing, he now finds himself trying to fulfil that early promise - maybe in what seems like a ultimate opportunity - under recent support.
He opened tentatively, with both competitors prepared to be aggressive in a cagey early stages.
Parker, whose sole prior loss 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 fight's midpoint, Parker had kept it close, causing problems for Buatsi, who operated with a tight guard to narrowly win the more effective periods.
The audience's tolerance wore thin amid regular grabbing, with the challenger going down various instances during close exchanges.
The underdog ended stronger, but many rounds were hard to judge. However, previous titleholder the boxing analyst - commentating on the broadcast network - had it particularly wide for the challenger, scoring for Buatsi just one round.
Buatsi moves on, but with the presence of hard hitter Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - on this evidence - still appears challenging.
Hatton Memorial Fuels Manchester Derby Contest
In the secondary fight, Lyndon Arthur defeated Bradley Rea by close verdict in a excellent all-Manchester contest to secure the continental championship.
The venue fell silent for a profoundly moving memorial prior to the bout as ten bells rang in memory of local hero Ricky Hatton, who succumbed 46 years old in September.
It was the premier televised boxing event in the urban center since the passing of Hatton. As the last bell faded, the quiet became a roar of "there's just one Hatton".
The fighting inside the ropes matched the emotion. The eventual winner was floored by a left hook squarely on the face in the initial stage, standing uncertainly to his position on the eight count.
But the red-colored shorts of Arthur soon pressed ahead again. On the opposite side, the opponent - in competitive blue - was damaged quickly, significant bruising developing under his eye as the duo traded punishing blows.
Rea, twenty-seven, landed power punches but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the intensity rarely dropped over twelve rounds.
The victor barely secured the result with scoring tallies of draw, 115-113 and wider gap, but the two athletes were persistent and courageous, exactly what their hero the legend stood for.
