UK light-heavyweight Buatsi earned a win yet disappointing beginning to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a judges' verdict in a competitive 10-round bout at the Manchester venue.
But Parker landed significant shots, connecting with powerful single shots in a fight where the styles never quite clicked, with excessive holding disrupting the action.
Two judges judged it 96-94 in favor to Buatsi, with the final judge a 95-95.
"It wasn't my greatest showing but how do champions respond? They prevail," the victor stated in the ring as he was faced disapproval from the audience.
The Londoner bounced back after his initial pro loss - an memorable fight with Callum Smith in last winter - to secure a twentieth professional win.
Several boxing experts believed Parker, 31, should have been awarded the decision but the British boxer experienced the second defeat of his 28-match professional journey.
The Champion Earns Triumph in Uneventful Encounter
Buatsi's progress has plateaued in the past few years through management issues and considerable time of inactivity.
Previously predicted as the next star of British boxing, he now is positioned trying to achieve expectations - possibly in what seems like a last chance - under recent support.
He began cautiously, with both competitors ready to overcommit in a cagey early stages.
Parker, whose single earlier setback came at the lower weight class against his former opponent in 2022 after damaging his hand during the bout, was competing in only his second fight at light heavyweight.
By the middle rounds, Parker had stayed in contention, causing problems for Buatsi, who worked behind a protective stance to narrowly win the more effective periods.
The crowd's patience diminished during frequent holding, with the challenger slipping to the canvas various instances during close exchanges.
Parker closed well, but several stanzas were difficult to score. Nevertheless, ex-champion the boxing analyst - working with the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just one round.
Buatsi progresses, but with the likes of hard hitter Benavidez, and Russian star Dmitry Bivol leading the category, the path to championship success - on this evidence - still appears challenging.
Ricky Hatton Homage Inspires Manchester Derby Contest
In the co-main event, Arthur overcame Bradley Rea by divided decision in a terrific all-Manchester battle to capture the continental championship.
The stadium grew hushed for a extremely emotional homage before the fight as ten bells tolled in remembrance of Manchester's own Hatton, who succumbed 46 years old in recently.
It was the first major televised fight night in the city since Hatton's passing. As the concluding sound faded, the silence transitioned to a chant of "only one Hatton".
The fighting inside the ropes matched the sentiment. Arthur was floored by a powerful punch directly to the chin in the initial stage, standing uncertainly to his feet on the count of eight.
But the red-colored shorts of the fighter soon pressed ahead again. From the other corner, Rea - in competitive blue - was marked up soon, noticeable damage developing under his eye as the fighters traded hard punches.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, fought fire with fire with stinging uppercuts as the intensity rarely dropped over the full distance.
Arthur edged the decision with judges scorecards of draw, 115-113 and 115-112, but the two athletes were relentless and brave, everything their idol Hatton embodied.
