UK light-heavyweight Buatsi earned a win yet unimpressive start to life under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a judges' verdict in a close 10-round bout at the Manchester venue.
But the challenger had his moments, landing some telling individual punches in a contest where the styles failed to mesh, with frequent clinching spoiling the contest.
The scoring judges had it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my finest showing but how do champions respond? They prevail," the victor said in the arena as he was met by negative reactions from the crowd.
The British fighter returned to winning ways after his first professional defeat - an memorable fight with his previous opponent in February - to claim a 20th career victory.
Various analysts believed the challenger, thirty-one, should have been awarded the win but the British boxer experienced the second loss of his 28-match career.
Buatsi Earns Victory in Dull Encounter
His career development has stalled in recent times through contract problems and long spells of inactivity.
Once tipped as the next star of UK boxing, he now finds himself trying to achieve expectations - perhaps in what seems like a final roll of the dice - under recent support.
He opened tentatively, with both competitors willing to take risks in a tentative initial rounds.
Parker, whose single earlier setback came at the lower weight class against John Ryder in 2022 after breaking his hand in the contest, was competing in only his second bout at the higher weight class.
By the fight's midpoint, Parker had made it competitive, creating difficulties for Buatsi, who utilized a solid defense to narrowly win the cleaner stanzas.
The audience's tolerance diminished because of constant clinching, with the challenger going down several times during grappling.
Parker finished the stronger, but many rounds were challenging to assess. Nevertheless, ex-champion the boxing analyst - working with the broadcast network - had it particularly wide for Parker, giving Buatsi just one round.
Buatsi moves on, but with the likes of hard hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the journey to titles - on this evidence - still appears challenging.
Hatton Memorial Inspires All-Manchester Fight
In the secondary fight, Lyndon Arthur defeated Rea by divided decision in a terrific all-Manchester battle to capture the continental belt.
The stadium became quiet for a extremely emotional memorial before the fight as ten bells tolled in remembrance of Manchester's own Ricky Hatton, who died aged 46 in September.
It was the initial significant broadcast fight night in the city since Hatton's death. As the concluding sound faded, the stillness gave way to a chant of "there's just one Ricky Hatton".
The action within the ring reflected the feeling. Arthur was knocked down by a powerful punch squarely on the face in the initial stage, standing uncertainly to his footing on the referee's count.
But the traditional red shorts of Arthur soon advanced again. Across the ring, Rea - in competitive blue - was injured quickly, heavy swelling forming under his eye as the duo traded punishing blows.
Rea, 27, delivered effective strikes but the champion, 34, fought fire with fire with powerful upward punches as the pace rarely dropped over twelve rounds.
Arthur edged the verdict with judges scorecards of 114-114, narrow lead and clearer margin, but both fighters were relentless and brave, everything their hero Hatton represented.
