UK light heavyweight Buatsi earned a win yet unimpressive start to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a split decision in a close 10-round contest at the Co-op Live Arena.
But the challenger landed significant shots, connecting with powerful single shots in a fight where the approaches failed to mesh, with excessive holding marring the contest.
Two judges scored it 96-94 to Buatsi, with the third a even score.
"It wasn't my best display but what do champions do? They succeed," the victor said in the ringside as he was faced disapproval from the spectators.
The Londoner bounced back after his initial pro loss - an memorable fight with Callum Smith in last winter - to claim a 20th career victory.
Some ringside observers felt the challenger, thirty-one, should have been awarded the win but the contender suffered the second setback of his 28-bout career.
Buatsi Earns Win in Slow Affair
His career development has plateaued in recent years through management issues and extended periods of inactivity.
Previously predicted as the future of British boxing, he now is positioned trying to fulfil that early promise - maybe in what feels like a ultimate opportunity - under fresh management.
He began cautiously, with the two boxers willing to take risks in a calculated initial rounds.
The challenger, whose sole prior loss came at super middleweight against his former opponent in 2022 after damaging his hand during the bout, was participating in his second appearance at the higher weight class.
By the halfway stage, the underdog had stayed in contention, creating difficulties for the favorite, who worked behind a tight guard to barely take the more effective rounds.
The audience's tolerance decreased because of frequent holding, with the challenger going down various instances during clinches.
Parker closed well, but several stanzas were challenging to assess. Nevertheless, former world champion Carl Frampton - working with Dazn - had it strongly leaning for the challenger, giving Buatsi just one round.
The victor moves on, but with the caliber of power puncher David Benavidez, and Eastern European fighter Bivol leading the category, the road to world glory - based on this performance - still appears challenging.
Ricky Hatton Memorial Inspires Manchester Derby Contest
In the co-main event, Arthur overcame Rea by close verdict in a excellent Manchester vs Manchester fight to win the continental belt.
The venue fell silent for a deeply poignant tribute prior to the bout as ten bells tolled in remembrance of local hero Ricky Hatton, who passed away at age 46 in recently.
It was the initial significant broadcast boxing event in the location since the passing of Hatton. As the last bell disappeared, the silence became a roar of "only one Ricky Hatton".
The competition inside the ropes equaled the emotion. Arthur was sent to the canvas by a powerful punch flush on the chin in the second round, standing uncertainly to his position on the referee's count.
But the Manchester United-red shorts of Arthur soon pressed ahead again. From the other corner, Rea - in opposing blue - was injured quickly, heavy swelling developing under his face as the duo exchanged powerful shots.
The challenger, 27, delivered effective strikes but the champion, thirty-four, fought fire with fire with effective uppercuts as the action hardly diminished over the full distance.
The victor barely secured the result with scoring tallies of even score, close margin and wider gap, but each competitor were relentless and full of heart, everything their inspiration the legend embodied.
