British light heavyweight Buatsi made a winning yet unimpressive beginning to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a judges' verdict in a competitive 10-round fight at the Manchester venue.
But the challenger found success, delivering effective individual punches in a match where the approaches didn't complement each other, with frequent clinching spoiling the action.
The scoring judges scored it 96-94 for to Buatsi, with the final judge a even score.
"It wasn't my greatest showing but what defines champions? They succeed," the victor said in the arena as he was met by negative reactions from the audience.
The British fighter got back to winning after his maiden career setback - an epic battle with his previous opponent in February - to earn a 20th career victory.
Some ringside observers thought the challenger, 31, deserved the victory but the Derbyshire fighter suffered the second defeat of his 28-fight professional journey.
Buatsi Earns Victory in Dull Encounter
His career development has plateaued in recent times through contract problems and long spells of inactivity.
Previously predicted as the next star of UK boxing, he now stands trying to fulfil that early promise - possibly in what seems like a final roll of the dice - under recent support.
He started carefully, with the two boxers ready to overcommit in a calculated opening few rounds.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in 2022 after damaging his hand during the bout, was participating in his second fight at the higher weight class.
By the halfway stage, the underdog had made it competitive, creating difficulties for Buatsi, who operated with a solid defense to edge the better stanzas.
The spectators' acceptance wore thin because of frequent holding, with the challenger falling to the floor multiple occasions during clinches.
Parker ended stronger, but many rounds were hard to judge. Nevertheless, ex-champion the boxing analyst - providing analysis for Dazn - had it significantly in favor for Parker, awarding Buatsi just one round.
Buatsi progresses, but with the presence of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the path to championship success - judging by this showing - still seems difficult.
Ricky Hatton Tribute Fuels All-Manchester Battle
In the supporting bout, Lyndon Arthur defeated Bradley Rea by divided decision in a excellent all-Manchester battle to capture the European light heavyweight belt.
The stadium grew hushed for a profoundly moving homage prior to the bout as a bell ceremony sounded in honor of local hero Ricky Hatton, who passed away aged 46 in last month.
It was the initial significant broadcast boxing event in the city since Hatton's passing. As the concluding sound ended, the quiet gave way to a chant of "there's just one Ricky Hatton".
The competition during the bout equaled the sentiment. Arthur was floored by a left hook flush on the jaw in the initial stage, rising shakily to his footing on the count of eight.
But the Manchester United-red shorts of Arthur soon advanced again. From the other corner, the opponent - in opposing blue - was marked up early, heavy swelling developing under his orbital area as the duo exchanged hard punches.
Rea, 27, delivered effective strikes but Arthur, 34, responded in kind with stinging uppercuts as the intensity hardly diminished over twelve rounds.
The victor narrowly won the result with official scores of even score, close margin and wider gap, but the two athletes were persistent and brave, all the qualities their inspiration the legend stood for.
