UK light-heavyweight Buatsi earned a win yet underwhelming debut to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a majority decision in a close ten-round fight at the boxing arena.
But the challenger landed significant shots, connecting with powerful clean blows in a match where the styles never quite clicked, with constant grabbing marring the action.
The scoring judges scored it 96-94 for to Buatsi, with the final judge a 95-95.
"It wasn't my greatest showing but what defines champions? They win," the victor said in the arena as he was met by disapproval from the audience.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with his previous opponent in last winter - to earn a twentieth professional win.
Several boxing experts thought Parker, 31, merited the decision but the Derbyshire fighter suffered the second setback of his 28-bout career.
The Champion Claims Triumph in Dull Match
Buatsi's progress has stalled in recent times through management issues and long spells of inactivity.
Once tipped as the next star of British boxing, he now is positioned trying to fulfil that early promise - maybe in what seems like a last chance - under recent support.
He began cautiously, with the two boxers prepared to be aggressive in a cagey early stages.
Parker, whose only previous defeat came at super-middleweight against his former opponent in 2022 after damaging his hand during the bout, was making just his second bout at light-heavyweight.
By the fight's midpoint, Parker had kept it close, causing problems for Buatsi, who operated with a solid defense to narrowly win the cleaner rounds.
The crowd's patience diminished because of constant clinching, with Parker falling to the floor several times during close exchanges.
The underdog ended stronger, but many rounds were challenging to assess. However, ex-champion Carl Frampton - providing analysis for Dazn - had it significantly in favor for Parker, scoring for Buatsi just one round.
Buatsi advances, but with the presence of power puncher David Benavidez, and Eastern European fighter Bivol dominating the weight class, the road to world glory - judging by this showing - still looks steep.
Hatton Memorial Motivates All-Manchester Contest
In the co-main event, Lyndon Arthur overcame Bradley Rea by divided decision in a excellent Manchester vs Manchester battle to capture the European light heavyweight title.
The stadium fell silent for a profoundly moving homage before the fight as ten bells rang in remembrance of local hero Ricky Hatton, who died aged 46 in recently.
It was the premier televised boxing event in the city since the passing of Hatton. As the final chime disappeared, the stillness transitioned to a roar of "there's just one Ricky Hatton".
The fighting during the bout equaled the sentiment. Arthur was floored by a significant blow flush on the jaw in the initial stage, standing uncertainly to his feet on the referee's count.
But the traditional red equipment of Arthur soon came forward again. On the opposite side, the opponent - in rival City blue - was injured soon, significant bruising forming under his face as the pair traded punishing blows.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, fought fire with fire with powerful upward punches as the pace rarely dropped over 12 rounds.
Arthur edged the result with judges scorecards of draw, narrow lead and clearer margin, but both fighters were persistent and courageous, all the qualities their hero the legend embodied.
