UK light heavyweight Buatsi earned a win yet unimpressive beginning to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a judges' verdict in a competitive ten-round fight at the Manchester venue.
But Parker landed significant shots, landing some telling single shots in a match where the approaches didn't complement each other, with excessive holding disrupting the action.
The scoring judges scored it 96-94 to the winner, with the remaining official a even score.
"It wasn't my greatest showing but what defines champions? They win," Buatsi said in the arena as he was greeted with disapproval from the crowd.
The Londoner got back to winning after his initial pro loss - an memorable fight with Callum Smith in last winter - to claim a 20th career victory.
Some ringside observers thought the challenger, 31, deserved the decision but the contender suffered the second loss of his 28-bout career.
The Champion Secures Win in Slow Match
Buatsi's progress has plateaued in the past few years through management issues and extended periods of inactivity.
Once tipped as the future of UK boxing, he now stands trying to achieve expectations - possibly in what appears to be a last chance - under recent support.
He started carefully, with neither fighter ready to take risks in a cagey initial rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in 2022 after damaging his hand mid-fight, was making just his second appearance at light heavyweight.
By the middle rounds, Parker had kept it close, making life awkward for Buatsi, who worked behind a tight guard to barely take the better rounds.
The crowd's patience wore thin during regular grabbing, with the challenger going down multiple occasions during clinches.
Parker finished the stronger, but several stanzas were difficult to score. Nevertheless, former world champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just one round.
The victor advances, but with the caliber of power puncher David Benavidez, and Russian champion Bivol dominating the weight class, the road to world glory - based on this performance - still seems difficult.
Hatton Tribute Fuels All-Manchester Battle
In the co-main event, Lyndon Arthur beat Rea by divided decision in a terrific local rivalry battle to secure the European light-heavyweight belt.
The venue grew hushed for a profoundly moving homage before the fight as ten bells tolled in remembrance of Manchester's own Ricky Hatton, who passed away 46 years old in recently.
It was the premier televised boxing event in the city since the passing of Hatton. As the concluding sound disappeared, the stillness transitioned to a shout of "there's only one Hatton".
The competition within the ring reflected the emotion. Arthur was floored by a left hook flush on the face in the early round, rising shakily to his position on the referee's count.
But the Manchester United-red shorts of the fighter soon advanced again. On the opposite side, Rea - in competitive blue - was marked up soon, significant bruising forming under his eye as the duo landed powerful shots.
Rea, 27, delivered effective strikes but Arthur, thirty-four, responded in kind with powerful upward punches as the pace hardly diminished over the full distance.
The victor narrowly won the result with official scores of even score, 115-113 and wider gap, but the two athletes were persistent and brave, all the qualities their inspiration Hatton represented.
