UK light heavyweight Buatsi secured a victory yet underwhelming start to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a split decision in a competitive 10-round bout at the Co-op Live Arena.
But Parker had his moments, landing some telling individual punches in a contest where the approaches failed to mesh, with excessive holding spoiling the contest.
The scoring judges scored it 96-94 for to Buatsi, with the remaining official a 95-95.
"It wasn't my finest showing but what defines champions? They succeed," Buatsi said in the ring as he was met by negative reactions from the crowd.
The British fighter bounced back after his maiden career setback - an thrilling contest with Callum Smith in February - to claim a 20th career victory.
Various analysts felt Parker, thirty-one, deserved the decision but the Derbyshire fighter endured the second setback of his 28-fight career.
The Champion Claims Triumph in Slow Encounter
His career development has slowed in the past few years through promotional disputes and long spells of inactivity.
Previously predicted as the rising talent of British boxing, he now stands trying to fulfil that early promise - maybe in what appears to be a final roll of the dice - under fresh management.
He began cautiously, with neither fighter prepared to overcommit in a cagey opening few rounds.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in 2022 after breaking his hand in the contest, was making just his second fight at the higher weight class.
By the fight's midpoint, the underdog had kept it close, creating difficulties for the favorite, who worked behind a tight guard to barely take the better periods.
The audience's tolerance diminished during frequent holding, with the challenger going down multiple occasions during grappling.
Parker closed well, but multiple periods were difficult to score. Although, ex-champion the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just one round.
Buatsi progresses, but with the presence of power puncher Benavidez, and Russian star Bivol ruling the division, the road to world glory - on this evidence - still appears challenging.
Ricky Hatton Tribute Fuels All-Manchester Fight
In the supporting bout, Lyndon Arthur beat Rea by split-decision in a outstanding local rivalry contest to capture the European light-heavyweight belt.
The stadium fell silent for a profoundly moving memorial before the fight as 10 bells sounded in memory of local hero Hatton, who passed away at age 46 in recently.
It was the first major televised boxing event in the city since Hatton's passing. As the last bell faded, the stillness transitioned to a roar of "only one Ricky Hatton".
The competition within the ring matched the emotion. The eventual winner was floored by a significant blow flush on the jaw in the initial stage, rising shakily to his position on the eight count.
But the traditional red equipment of the fighter soon advanced again. Across the ring, Rea - in rival City blue - was damaged quickly, significant bruising developing under his face as the duo landed punishing blows.
The challenger, 27, delivered effective strikes but Arthur, 34, responded in kind with effective uppercuts as the action seldom decreased over 12 rounds.
The victor narrowly won the result with official scores of even score, narrow lead and clearer margin, but each competitor were determined and courageous, everything their hero Hatton stood for.
