UK light heavyweight Buatsi secured a victory yet disappointing start to his career under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a split decision in a competitive ten-round bout at the Manchester venue.
But Parker had his moments, landing some telling single shots in a contest where the techniques failed to mesh, with frequent clinching marring the contest.
The officials scored it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my best performance but how do champions respond? They win," Buatsi said in the ring as he was faced negative reactions from the spectators.
The Londoner returned to winning ways after his maiden career setback - an epic battle with his previous opponent in last winter - to claim a 20th pro success.
Several boxing experts felt the challenger, 31, merited the win but the contender endured the second defeat of his 28-match professional journey.
The Champion Secures Victory in Uneventful Encounter
Buatsi's progress has stalled in the past few years through contract problems and extended periods of lack of fights.
Once tipped as the future of UK boxing, he now is positioned trying to achieve expectations - perhaps in what seems like a final roll of the dice - under fresh management.
He started carefully, with the two boxers willing to overcommit in a cagey initial rounds.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in 2022 after damaging his hand in the contest, was competing in only his second fight at light heavyweight.
By the fight's midpoint, Parker had made it competitive, creating difficulties for Buatsi, who worked behind a protective stance to edge the cleaner stanzas.
The spectators' acceptance decreased amid constant clinching, with the challenger going down multiple occasions during close exchanges.
Parker closed well, but multiple periods were difficult to score. However, ex-champion the boxing analyst - working with the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just only one stanza.
The victor progresses, but with the likes of heavy hitter David Benavidez, and Russian star Bivol ruling the division, the path to championship success - based on this performance - still appears challenging.
Ricky Hatton Homage Motivates Manchester Derby Contest
In the secondary fight, Arthur beat Bradley Rea by split-decision in a outstanding local rivalry battle to capture the continental title.
The venue fell silent for a extremely emotional memorial before the fight as 10 bells sounded in honor of Manchester's own Ricky Hatton, who passed away at age 46 in last month.
It was the initial significant broadcast boxing event in the city since Hatton's passing. As the final chime faded, the stillness transitioned to a chant of "there's just one Hatton".
The competition within the ring reflected the emotion. The eventual winner was sent to the canvas by a significant blow directly to the face in the initial stage, rising shakily to his footing on the count of eight.
But the traditional red shorts of Arthur soon pressed ahead again. Across the ring, Rea - in rival City blue - was damaged quickly, heavy swelling forming under his face as the pair exchanged powerful shots.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, fought fire with fire with powerful upward punches as the action seldom decreased over the full distance.
Arthur barely secured the result with official scores of even score, 115-113 and wider gap, but each competitor were relentless and courageous, all the qualities their inspiration Hatton embodied.
