British light heavyweight Buatsi secured a victory yet underwhelming debut to his career under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a competitive ten-round contest at the Co-op Live Arena.
But Parker landed significant shots, delivering effective individual punches in a contest where the techniques failed to mesh, with frequent clinching spoiling the fight.
Two judges had it 96-94 in favor to the winner, with the third a 95-95.
"It wasn't my best showing but what defines champions? They succeed," Buatsi commented in the arena as he was greeted with negative reactions from the crowd.
The British fighter bounced back after his first professional defeat - an thrilling contest with Callum Smith in February - to earn a 20th career victory.
Some ringside observers believed the challenger, 31, deserved the win but the contender suffered the second loss of his 28-match career.
Buatsi Claims Victory in Uneventful Match
His career development has stalled in recent times through management issues and long spells of inactivity.
Previously predicted as the future of the British fight scene, he now is positioned trying to realize his potential - possibly in what seems like a last chance - under fresh management.
He opened tentatively, with both competitors ready to take risks in a calculated opening few rounds.
Parker, whose single earlier setback came at the lower weight class against his former opponent in two years ago after breaking his hand mid-fight, was competing in only his second fight at light-heavyweight.
By the middle rounds, Parker had kept it close, causing problems for Buatsi, who operated with a solid defense to edge the cleaner periods.
The audience's tolerance diminished amid frequent holding, with Parker falling to the floor various instances during close exchanges.
The underdog closed well, but multiple periods were hard to judge. Although, ex-champion Carl Frampton - commentating on Dazn - had it particularly wide for the challenger, giving Buatsi just only one stanza.
Buatsi advances, but with the presence of heavy hitter Benavidez, and Russian champion Bivol leading the category, the journey to titles - on this evidence - still looks steep.
Hatton Memorial Fuels Manchester Derby Battle
In the secondary fight, Lyndon Arthur beat Rea by divided decision in a excellent all-Manchester contest to win the continental championship.
The venue fell silent for a deeply poignant homage ahead of the contest as 10 bells rang in honor of local hero Hatton, who died 46 years old in September.
It was the initial significant broadcast match card in the location since Hatton's passing. As the concluding sound faded, the stillness transitioned to a chant of "there's just one Ricky Hatton".
The fighting during the bout matched the emotion. The eventual winner was sent to the canvas by a powerful punch directly to the chin in the initial stage, standing uncertainly to his position on the count of eight.
But the Manchester United-red equipment of Arthur soon came forward again. Across the ring, Rea - in competitive blue - was marked up soon, significant bruising appearing under his face as the duo landed punishing blows.
Rea, twenty-seven, landed power punches but Arthur, 34, responded in kind with stinging uppercuts as the action rarely dropped over twelve rounds.
Arthur edged the verdict with official scores of draw, narrow lead and wider gap, but the two athletes were determined and courageous, everything their inspiration Hatton stood for.
