British light-heavyweight Buatsi made a winning yet disappointing debut to life under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a majority decision in a tight 10-round contest at the Co-op Live Arena.
But the challenger had his moments, connecting with powerful clean blows in a contest where the approaches didn't complement each other, with frequent clinching marring the contest.
The scoring judges had it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my finest showing but what defines champions? They succeed," the victor said in the arena as he was greeted with negative reactions from the crowd.
The Londoner bounced back after his first professional defeat - an thrilling contest with his previous opponent in February - to earn a 20th career victory.
Various analysts felt Parker, 31, should have been awarded the win but the contender experienced the second loss of his 28-fight professional journey.
The Champion Secures Triumph in Uneventful Encounter
His career development has stalled in recent times through promotional disputes and considerable time of lack of fights.
Earlier forecast as the future of British boxing, he now is positioned trying to fulfil that early promise - maybe in what feels like a last chance - under recent support.
He began cautiously, with both competitors prepared to overcommit in a tentative opening few rounds.
The challenger, whose sole prior loss came at super middleweight against John Ryder in 2022 after suffering a hand injury during the bout, was participating in his second bout at light heavyweight.
By the middle rounds, Parker had stayed in contention, causing problems for the favorite, who operated with a protective stance to barely take the more effective periods.
The audience's tolerance diminished because of constant clinching, with Parker slipping to the canvas various instances during close exchanges.
Parker finished the stronger, but several stanzas were difficult to score. Nevertheless, former world champion Carl Frampton - providing analysis for Dazn - had it strongly leaning for Parker, scoring for Buatsi just a single round.
The victor progresses, but with the caliber of heavy hitter Benavidez, and Russian star Dmitry Bivol ruling the division, the road to world glory - based on this performance - still appears challenging.
Ricky Hatton Memorial Fuels Local Rivalry Fight
In the supporting bout, Arthur defeated Bradley Rea by close verdict in a outstanding all-Manchester fight to win the continental championship.
The stadium became quiet for a extremely emotional tribute prior to the bout as ten bells sounded in honor of Manchester's own Ricky Hatton, who died aged 46 in last month.
It was the premier televised boxing event in the urban center since the passing of Hatton. As the concluding sound ended, the stillness transitioned to a chant of "only one Hatton".
The fighting during the bout matched the feeling. Arthur was sent to the canvas by a powerful punch squarely on the chin in the early round, standing uncertainly to his footing on the count of eight.
But the traditional red attire of Arthur soon pressed ahead again. On the opposite side, the opponent - in opposing blue - was damaged quickly, significant bruising appearing under his orbital area as the pair exchanged punishing blows.
The challenger, 27, delivered effective strikes but Arthur, thirty-four, matched the intensity with stinging uppercuts as the pace hardly diminished over 12 rounds.
The victor narrowly won the verdict with scoring tallies of even score, 115-113 and clearer margin, but the two athletes were relentless and brave, all the qualities their hero the legend embodied.
