British light-heavyweight Buatsi made a winning yet disappointing beginning to life under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a split decision in a close ten-round contest at the boxing arena.
But the challenger had his moments, delivering effective individual punches in a match where the styles didn't complement each other, with excessive holding disrupting the contest.
The officials judged it 96-94 to Buatsi, with the third a even score.
"It wasn't my greatest showing but how do champions respond? They win," Buatsi commented in the arena as he was met by disapproval from the audience.
The British fighter got back to winning after his maiden career setback - an memorable fight with Callum Smith in last winter - to secure a 20th pro success.
Some ringside observers felt the challenger, 31, deserved the decision but the British boxer experienced the second defeat of his 28-bout professional journey.
The Champion Secures Victory in Slow Match
The fighter's advancement has stalled in the past few years through management issues and extended periods of inactivity.
Previously predicted as the next star of British boxing, he now is positioned trying to achieve expectations - perhaps in what seems like a final roll of the dice - under recent support.
He started carefully, with both competitors ready to be aggressive in a calculated opening few rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in two years ago after damaging his hand mid-fight, was making just his second bout at light-heavyweight.
By the fight's midpoint, the underdog had stayed in contention, causing problems for the favorite, who worked behind a solid defense to edge the better stanzas.
The spectators' acceptance diminished during frequent holding, with the challenger falling to the floor multiple occasions during grappling.
The underdog finished the stronger, but many rounds were difficult to score. Nevertheless, previous titleholder Carl Frampton - working with Dazn - had it significantly in favor for Parker, giving Buatsi just only one stanza.
Buatsi progresses, but with the presence of power puncher Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the path to championship success - based on this performance - still looks steep.
Ricky Hatton Tribute Motivates Manchester Derby Fight
In the co-main event, Lyndon Arthur overcame Rea by close verdict in a excellent all-Manchester fight to capture the continental belt.
The venue grew hushed for a deeply poignant memorial before the fight as 10 bells tolled in honor of the city's native Ricky Hatton, who died at age 46 in September.
It was the initial significant broadcast fight night in the urban center since Hatton's death. As the final chime ended, the stillness gave way to a chant of "there's only one Ricky Hatton".
The fighting within the ring matched the emotion. The eventual winner was knocked down by a significant blow squarely on the jaw in the early round, rising shakily to his position on the eight count.
But the traditional red attire of Arthur soon came forward again. From the other corner, Rea - in opposing blue - was injured soon, heavy swelling developing under his face as the duo landed hard punches.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, matched the intensity with stinging uppercuts as the intensity seldom decreased over 12 rounds.
The victor barely secured the verdict with official scores of 114-114, close margin and wider gap, but each competitor were relentless and courageous, everything their hero the legend represented.
