UK light-heavyweight Buatsi earned a win yet underwhelming debut to life under recent management 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 close 10-round fight at the Co-op Live Arena.
But Parker landed significant shots, landing some telling individual punches in a match where the techniques failed to mesh, with frequent clinching marring the fight.
The scoring judges scored it 96-94 in favor to the winner, with the remaining official a 95-95 draw.
"It wasn't my best showing but what do champions do? They succeed," the victor said in the ringside as he was faced negative reactions from the audience.
The British fighter got back to winning after his maiden career setback - an epic battle with Callum Smith in last winter - to earn a 20th career victory.
Some ringside observers thought the challenger, 31, deserved the victory but the British boxer endured the second loss of his 28-bout career.
The Champion Secures Win in Slow Affair
Buatsi's progress has plateaued in recent times 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 achieve expectations - perhaps in what seems like a last chance - under new promotional backing.
He started carefully, with both competitors ready to overcommit in a tentative initial rounds.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was participating in his second appearance at the higher weight class.
By the middle rounds, the underdog had stayed in contention, making life awkward for the favorite, who worked behind a solid defense to barely take the more effective periods.
The spectators' acceptance decreased during constant clinching, with Parker falling to the floor multiple occasions during close exchanges.
The underdog closed well, but multiple periods were hard to judge. Although, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for the challenger, giving Buatsi just a single round.
Buatsi advances, but with the likes of heavy hitter David Benavidez, and Russian champion Bivol ruling the division, the journey to titles - judging by this showing - still appears challenging.
Hatton Tribute Motivates Local Rivalry Contest
In the secondary fight, Lyndon Arthur overcame Bradley Rea by divided decision in a terrific local rivalry fight to win the continental championship.
The stadium grew hushed for a extremely emotional homage before the fight as a bell ceremony tolled in remembrance of local hero Ricky Hatton, who passed away 46 years old in September.
It was the premier televised fight night in the location since Hatton's passing. As the last bell disappeared, the quiet became a chant of "there's just one Hatton".
The action inside the ropes equaled the emotion. The eventual winner was sent to the canvas by a left hook directly to the chin in the early round, standing uncertainly to his footing on the referee's count.
But the traditional red shorts of Arthur soon advanced again. From the other corner, the opponent - in competitive blue - was damaged soon, heavy swelling forming under his eye as the duo landed powerful shots.
Rea, 27, connected with strong blows but the champion, 34, responded in kind with effective uppercuts as the action seldom decreased over 12 rounds.
Arthur edged the verdict with official scores of 114-114, 115-113 and 115-112, but both fighters were relentless and brave, exactly what their inspiration Hatton embodied.
