British light heavyweight Buatsi secured a victory yet underwhelming debut to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a judges' verdict in a close 10-round fight at the Co-op Live Arena.
But Parker found success, connecting with powerful individual punches in a contest where the styles didn't complement each other, with constant grabbing disrupting the contest.
Two judges had it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my finest performance but what defines champions? They prevail," Buatsi said in the ring as he was greeted with disapproval from the audience.
The Londoner got back to winning after his maiden career setback - an thrilling contest with his previous opponent in February - to claim a twentieth professional win.
Various analysts felt Parker, 31, deserved the victory but the Derbyshire fighter experienced the second setback of his 28-bout professional journey.
The Champion Earns Win in Uneventful Affair
The fighter's advancement has plateaued in the past few years through management issues and considerable time of inactivity.
Once tipped as the next star of British boxing, he now finds himself trying to realize his potential - possibly in what appears to be a ultimate opportunity - under new promotional backing.
He opened tentatively, with the two boxers prepared to overcommit in a cagey opening few rounds.
Parker, whose single earlier setback came at super middleweight against his former opponent in 2022 after breaking his hand mid-fight, was competing in only his second appearance at the higher weight class.
By the halfway stage, Parker had kept it close, making life awkward for the favorite, who worked behind a protective stance to barely take the cleaner periods.
The crowd's patience decreased amid regular grabbing, with Parker slipping to the canvas various instances during close exchanges.
The underdog finished the stronger, but multiple periods were challenging to assess. Nevertheless, previous titleholder the boxing analyst - commentating on Dazn - had it significantly in favor for Parker, awarding Buatsi just only one stanza.
The victor progresses, but with the caliber of heavy hitter Benavidez, and Russian champion Dmitry Bivol ruling the division, the journey to titles - on this evidence - still looks steep.
Hatton Homage Motivates All-Manchester Contest
In the secondary fight, Arthur overcame Bradley Rea by split-decision in a outstanding local rivalry contest to secure the continental belt.
The arena grew hushed for a extremely emotional memorial ahead of the contest as a bell ceremony sounded in memory of Manchester's own Hatton, who passed away 46 years old in recently.
It was the initial significant broadcast fight night in the city since the passing of Hatton. As the final chime ended, the stillness gave way to a shout of "there's only one Hatton".
The competition inside the ropes matched the feeling. Arthur was floored by a left hook squarely on the chin in the early round, standing uncertainly to his position on the eight count.
But the red-colored equipment of Arthur soon pressed ahead again. From the other corner, the opponent - in competitive blue - was marked up quickly, noticeable damage forming under his orbital area as the pair landed hard punches.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with stinging uppercuts as the pace hardly diminished over the full distance.
The victor narrowly won the verdict with judges scorecards of even score, 115-113 and 115-112, but both fighters were persistent and brave, everything their inspiration Hatton stood for.
