UK light-heavyweight Buatsi secured a victory yet underwhelming debut to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a judges' verdict in a close 10-round bout at the boxing arena.
But the challenger found success, connecting with powerful single shots in a contest where the techniques didn't complement each other, with frequent clinching marring the fight.
Two judges judged it 96-94 in favor to the winner, with the final judge a 95-95.
"It wasn't my best display but what do champions do? They win," the victor commented in the ring as he was faced boos from the audience.
The British fighter returned to winning ways after his first professional defeat - an epic battle with his previous opponent in last winter - to secure a 20th pro success.
Several boxing experts felt the challenger, 31, merited the decision but the British boxer endured the second loss of his 28-bout career.
The Champion Earns Triumph in Slow Encounter
His career development has plateaued in the past few years through promotional disputes and considerable time of lack of fights.
Earlier forecast as the next star of British boxing, he now is positioned trying to realize his potential - maybe in what feels like a last chance - under fresh management.
He started carefully, with the two boxers ready to take risks in a calculated initial rounds.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in 2022 after breaking his hand mid-fight, was competing in only his second bout at light heavyweight.
By the fight's midpoint, Parker had kept it close, creating difficulties for Buatsi, who utilized a protective stance to narrowly win the cleaner stanzas.
The spectators' acceptance wore thin during regular grabbing, with Parker slipping to the canvas multiple occasions during close exchanges.
Parker finished the stronger, but several stanzas were challenging to assess. Nevertheless, ex-champion the boxing analyst - commentating on the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just only one stanza.
Buatsi advances, but with the presence of heavy hitter David Benavidez, and Russian star Bivol ruling the division, the path to championship success - judging by this showing - still seems difficult.
Ricky Hatton Tribute Fuels All-Manchester Fight
In the supporting bout, Lyndon Arthur beat Bradley Rea by close verdict in a terrific local rivalry fight to win the European light-heavyweight belt.
The stadium grew hushed for a extremely emotional memorial prior to the bout as ten bells sounded in honor of the city's native Ricky Hatton, who succumbed aged 46 in September.
It was the premier televised boxing event in the city since Hatton's death. As the concluding sound faded, the stillness transitioned to a roar of "there's just one Hatton".
The action inside the ropes reflected the emotion. Arthur was floored by a left hook squarely on the jaw in the second round, rising shakily to his feet on the count of eight.
But the traditional red attire of the fighter soon came forward again. From the other corner, the opponent - in rival City blue - was damaged early, significant bruising developing under his face as the duo landed hard punches.
The challenger, twenty-seven, landed power punches but Arthur, thirty-four, responded in kind with stinging uppercuts as the action seldom decreased over 12 rounds.
The victor narrowly won the result with official scores of even score, close margin and wider gap, but both fighters were relentless and brave, exactly what their hero the legend embodied.
