British light-heavyweight Buatsi secured a victory yet unimpressive debut to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a judges' verdict in a tight ten-round bout at the Co-op Live Arena.
But the challenger found success, connecting with powerful single shots in a fight where the techniques didn't complement each other, with excessive holding spoiling the fight.
Two judges scored it 96-94 for to Buatsi, with the final judge a 95-95.
"It wasn't my finest display but how do champions respond? They win," the victor said in the ringside as he was greeted with disapproval from the spectators.
The British fighter got back to winning after his maiden career setback - an thrilling contest with Callum Smith in February - to earn a 20th career victory.
Some ringside observers thought Parker, thirty-one, should have been awarded the win but the Derbyshire fighter endured the second loss of his 28-bout professional journey.
Buatsi Claims Win in Slow Affair
The fighter's advancement has plateaued in recent times through promotional disputes and extended periods of inactivity.
Previously predicted as the next star of UK boxing, he now finds himself trying to fulfil that early promise - maybe in what appears to be a last chance - under recent support.
He opened tentatively, with both competitors prepared to overcommit in a calculated initial rounds.
The challenger, whose single earlier setback came at super middleweight against his former opponent in 2022 after damaging his hand mid-fight, was making just his second appearance at light heavyweight.
By the middle rounds, the underdog had kept it close, making life awkward for Buatsi, who utilized a solid defense to edge the cleaner stanzas.
The spectators' acceptance diminished amid regular grabbing, with the challenger falling to the floor several times during close exchanges.
Parker finished the stronger, but several stanzas were hard to judge. Nevertheless, previous titleholder the boxing analyst - working with Dazn - had it strongly leaning for Parker, giving Buatsi just one round.
Buatsi progresses, but with the likes of hard hitter Benavidez, and Russian star Dmitry Bivol dominating the weight class, the path to championship success - judging by this showing - still seems difficult.
Ricky Hatton Memorial Motivates All-Manchester Battle
In the secondary fight, Lyndon Arthur defeated Rea by split-decision in a outstanding all-Manchester contest to secure the European light heavyweight championship.
The stadium became quiet for a deeply poignant tribute before the fight as ten bells tolled in remembrance of the city's native Hatton, who died aged 46 in recently.
It was the first major televised fight night in the urban center since Hatton's passing. As the final chime faded, the stillness became a chant of "there's only one Ricky Hatton".
The competition during the bout equaled the sentiment. Arthur was knocked down by a significant blow directly to the chin in the second round, standing uncertainly to his footing on the count of eight.
But the Manchester United-red shorts of Arthur soon advanced again. From the other corner, Rea - in opposing blue - was damaged early, heavy swelling forming under his eye as the duo landed hard punches.
The challenger, 27, delivered effective strikes but Arthur, 34, responded in kind with effective uppercuts as the intensity hardly diminished over the full distance.
Arthur narrowly won the verdict with judges scorecards of even score, close margin and wider gap, but the two athletes were determined and courageous, everything their hero the legend stood for.
