UK light-heavyweight Buatsi secured a victory yet disappointing beginning to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a judges' verdict in a close ten-round bout at the Manchester venue.
But the challenger had his moments, connecting with powerful single shots in a match where the techniques didn't complement each other, with constant grabbing marring the fight.
Two judges scored it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my finest display but what do champions do? They prevail," the victor commented in the ringside as he was faced negative reactions from the crowd.
The British fighter got back to winning after his initial pro loss - an thrilling contest with Callum Smith in February - to claim a 20th pro success.
Several boxing experts felt Parker, thirty-one, deserved the decision but the contender experienced the second setback of his 28-bout career.
The Champion Earns Triumph in Dull Affair
The fighter's advancement has slowed in the past few years through management issues and considerable time of lack of fights.
Once tipped as the rising talent of British boxing, he now finds himself trying to realize his potential - perhaps in what feels like a final roll of the dice - under fresh management.
He began cautiously, with both competitors prepared to overcommit in a tentative initial rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after breaking his hand during the bout, was making just his second appearance at the higher weight class.
By the halfway stage, the underdog had stayed in contention, causing problems for Buatsi, who worked behind a protective stance to narrowly win the more effective stanzas.
The audience's tolerance diminished amid frequent holding, with the challenger falling to the floor multiple occasions during close exchanges.
The underdog closed well, but multiple periods were challenging to assess. Although, former world champion the boxing analyst - providing analysis for the broadcast network - had it significantly in favor for the challenger, giving Buatsi just a single round.
Buatsi advances, but with the likes of heavy hitter Benavidez, and Russian star Dmitry Bivol leading the category, the road to world glory - based on this performance - still seems difficult.
Hatton Tribute Inspires Local Rivalry Battle
In the secondary fight, Arthur defeated Bradley Rea by close verdict in a terrific all-Manchester battle to secure the continental belt.
The venue fell silent for a extremely emotional memorial prior to the bout as 10 bells sounded in remembrance of local hero Ricky Hatton, who died aged 46 in September.
It was the premier televised boxing event in the city since Hatton's passing. As the concluding sound ended, the quiet gave way to a roar of "there's just one Hatton".
The competition during the bout equaled the feeling. Arthur was sent to the canvas by a left hook directly to the face in the early round, rising shakily to his footing on the eight count.
But the red-colored attire of Arthur soon pressed ahead again. Across the ring, Rea - in opposing blue - was injured quickly, significant bruising forming under his eye as the duo traded powerful shots.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, matched the intensity with stinging uppercuts as the pace hardly diminished over the full distance.
The victor barely secured the verdict with scoring tallies of even score, 115-113 and clearer margin, but the two athletes were relentless and full of heart, exactly what their hero the legend stood for.
