UK light heavyweight Buatsi secured a victory yet disappointing start to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a split decision in a tight 10-round contest at the Co-op Live Arena.
But the challenger had his moments, landing some telling single shots in a contest where the approaches didn't complement each other, with constant grabbing marring the fight.
The scoring judges scored it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my best showing but how do champions respond? They win," Buatsi commented in the ring as he was faced negative reactions from the spectators.
The British fighter bounced back after his maiden career setback - an epic battle with his previous opponent in last winter - to claim a 20th career victory.
Several boxing experts felt the challenger, 31, should have been awarded the decision but the British boxer endured the second defeat of his 28-fight professional journey.
Buatsi Earns Win in Uneventful Encounter
His career development has slowed in recent times through contract problems and extended periods of inactivity.
Earlier forecast as the next star of UK boxing, he now is positioned trying to fulfil that early promise - perhaps in what seems like a last chance - under new promotional backing.
He began cautiously, with both competitors willing to take risks in a cagey initial rounds.
Parker, whose sole prior loss came at the lower weight class against his former opponent in 2022 after breaking his hand during the bout, was making just his second appearance at light heavyweight.
By the halfway stage, the underdog had made it competitive, making life awkward for Buatsi, who utilized a protective stance to barely take the more effective stanzas.
The spectators' acceptance decreased amid regular grabbing, with Parker slipping to the canvas various instances during clinches.
Parker finished the stronger, but many rounds were difficult to score. Nevertheless, previous titleholder Carl Frampton - commentating on Dazn - had it strongly leaning for Parker, scoring for Buatsi just a single round.
Buatsi progresses, but with the likes of power puncher Benavidez, and Russian star Dmitry Bivol dominating the weight class, the path to championship success - judging by this showing - still appears challenging.
Hatton Memorial Inspires Local Rivalry Contest
In the secondary fight, Arthur overcame Rea by close verdict in a terrific all-Manchester battle to win the European light-heavyweight belt.
The stadium became quiet for a deeply poignant memorial prior to the bout as ten bells sounded in remembrance of the city's native Ricky Hatton, who died 46 years old in last month.
It was the initial significant broadcast match card in the location since Hatton's passing. As the last bell ended, the quiet became a shout of "there's only one Ricky Hatton".
The fighting within the ring reflected the sentiment. Arthur was knocked down by a powerful punch directly to the chin in the initial stage, getting up unsteadily to his footing on the referee's count.
But the traditional red attire of Arthur soon advanced again. On the opposite side, Rea - in rival City blue - was injured quickly, noticeable damage forming under his eye as the pair traded hard punches.
The challenger, 27, delivered effective strikes but the champion, 34, matched the intensity with effective uppercuts as the intensity seldom decreased over 12 rounds.
Arthur narrowly won the verdict with official scores of 114-114, close margin and 115-112, but the two athletes were relentless and brave, all the qualities their idol the legend represented.
