UK light-heavyweight Buatsi earned a win yet unimpressive beginning to life under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a close 10-round contest at the Co-op Live Arena.
But the challenger had his moments, connecting with powerful single shots in a match where the styles didn't complement each other, with constant grabbing disrupting the action.
The scoring judges scored it 96-94 for to the winner, with the third a 95-95.
"It wasn't my greatest performance but how do champions respond? They win," the victor commented in the ringside as he was greeted with boos from the crowd.
The Londoner returned to winning ways after his first professional defeat - an thrilling contest with his previous opponent in February - to claim a 20th pro success.
Various analysts believed the challenger, 31, should have been awarded the decision but the Derbyshire fighter endured the second defeat of his 28-fight career.
Buatsi Secures Victory in Slow Affair
The fighter's advancement has plateaued in recent times through contract problems and long spells of inactivity.
Earlier forecast as the rising talent of UK boxing, he now finds himself trying to achieve expectations - maybe in what feels like a final roll of the dice - under fresh management.
He began cautiously, with the two boxers prepared to take risks in a calculated initial rounds.
Parker, whose single earlier setback came at the lower weight class against his former opponent in 2022 after suffering a hand injury mid-fight, was competing in only his second fight at light heavyweight.
By the middle rounds, the underdog had made it competitive, creating difficulties for Buatsi, who worked behind a tight guard to barely take the more effective periods.
The crowd's patience wore thin amid regular grabbing, with Parker going down several times during close exchanges.
Parker finished the stronger, but many rounds were challenging to assess. Although, previous titleholder Carl Frampton - providing analysis for Dazn - had it strongly leaning for Parker, scoring for Buatsi just a single round.
The victor advances, but with the likes of hard hitter Benavidez, and Russian star Bivol dominating the weight class, the road to world glory - on this evidence - still appears challenging.
Hatton Homage Inspires All-Manchester Battle
In the secondary fight, Arthur beat Bradley Rea by close verdict in a excellent Manchester vs Manchester fight to capture the continental belt.
The venue became quiet for a extremely emotional tribute before the fight as a bell ceremony rang in memory of local hero Ricky Hatton, who passed away at age 46 in last month.
It was the first major televised fight night in the city since the passing of Hatton. As the final chime faded, the silence transitioned to a roar of "only one Hatton".
The competition during the bout equaled the emotion. Arthur was floored by a left hook directly to the face in the early round, standing uncertainly to his feet on the eight count.
But the traditional red attire of Arthur soon advanced again. Across the ring, Rea - in opposing blue - was damaged quickly, noticeable damage appearing under his eye as the fighters exchanged punishing blows.
Rea, 27, delivered effective strikes but the champion, thirty-four, fought fire with fire with effective uppercuts as the intensity hardly diminished over the full distance.
The victor barely secured the decision with official scores of draw, narrow lead and 115-112, but the two athletes were persistent and full of heart, everything their inspiration Hatton embodied.
