UK light heavyweight Buatsi earned a win yet unimpressive debut to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a split decision in a competitive ten-round bout at the Co-op Live Arena.
But Parker had his moments, delivering effective single shots in a contest where the styles failed to mesh, with constant grabbing marring the contest.
Two judges had it 96-94 for to the winner, with the final judge a 95-95.
"It wasn't my best performance but how do champions respond? They succeed," Buatsi commented in the arena as he was met by negative reactions from the audience.
The British fighter bounced back after his maiden career setback - an memorable fight with Callum Smith in last winter - to earn a 20th pro success.
Some ringside observers believed the challenger, 31, deserved the win but the contender endured the second loss of his 28-match career.
Buatsi Claims Win in Slow Affair
Buatsi's progress has slowed in recent years through management issues and considerable time of lack of fights.
Previously predicted as the next star of British boxing, he now stands trying to achieve expectations - maybe in what appears to be a ultimate opportunity - under new promotional backing.
He began cautiously, with the two boxers ready to overcommit in a cagey early stages.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in 2022 after breaking his hand during the bout, was making just his second appearance at the higher weight class.
By the fight's midpoint, the underdog had kept it close, making life awkward for Buatsi, who operated with a tight guard to barely take the cleaner periods.
The spectators' acceptance diminished amid constant clinching, with the challenger going down multiple occasions during close exchanges.
Parker closed well, but many rounds were hard to judge. Nevertheless, ex-champion Carl Frampton - commentating on Dazn - had it strongly leaning for the challenger, awarding Buatsi just only one stanza.
The victor progresses, but with the presence of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the road to world glory - on this evidence - still seems difficult.
Hatton Tribute Inspires Manchester Derby Contest
In the co-main event, Lyndon Arthur overcame Rea by close verdict in a outstanding local rivalry fight to capture the continental title.
The arena fell silent for a deeply poignant tribute before the fight as a bell ceremony rang in remembrance of Manchester's own Hatton, who passed away at age 46 in recently.
It was the first major televised match card in the city since Hatton's passing. As the final chime faded, the silence transitioned to a roar of "only one Ricky Hatton".
The action during the bout equaled the sentiment. Arthur was knocked down by a significant blow flush on the chin in the early round, standing uncertainly to his feet on the eight count.
But the traditional red shorts of the fighter soon advanced again. Across the ring, Rea - in competitive blue - was injured quickly, heavy swelling forming under his orbital area as the duo exchanged hard punches.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, fought fire with fire with effective uppercuts as the intensity seldom decreased over 12 rounds.
The victor barely secured the decision with scoring tallies of 114-114, narrow lead and 115-112, but both fighters were relentless and brave, everything their idol Hatton represented.
