British light-heavyweight Buatsi earned a win yet disappointing debut to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's activity level seemingly secured him a judges' verdict in a tight 10-round contest at the Co-op Live Arena.
But Parker landed significant shots, delivering effective individual punches in a match where the approaches never quite clicked, with constant grabbing spoiling the fight.
The officials judged it 96-94 for to Buatsi, with the final judge a 95-95 draw.
"It wasn't my best performance but what defines champions? They prevail," the victor stated in the arena as he was met by disapproval from the crowd.
The Londoner returned to winning ways after his first professional defeat - an thrilling contest with his previous opponent in last winter - to secure a 20th pro success.
Some ringside observers believed Parker, thirty-one, deserved the victory but the British boxer suffered the second setback of his 28-match professional journey.
The Champion Secures Win in Dull Affair
His career development has slowed in recent years through management issues and long spells of inactivity.
Previously predicted as the next star of UK boxing, he now finds himself trying to achieve expectations - possibly in what feels like a last chance - under new promotional backing.
He began cautiously, with neither fighter willing to overcommit in a calculated initial rounds.
The challenger, whose single earlier setback came at super middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was making just his second bout at light heavyweight.
By the halfway stage, Parker had stayed in contention, making life awkward for Buatsi, who utilized a tight guard to edge the better rounds.
The spectators' acceptance wore thin because of regular grabbing, with the challenger slipping to the canvas several times during close exchanges.
Parker finished the stronger, but multiple periods were hard to judge. Although, ex-champion the boxing analyst - commentating on the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just only one stanza.
Buatsi moves on, but with the caliber of power puncher David Benavidez, and Eastern European fighter Bivol dominating the weight class, the road to world glory - based on this performance - still looks steep.
Ricky Hatton Tribute Motivates Manchester Derby Fight
In the supporting bout, Lyndon Arthur defeated Rea by close verdict in a excellent all-Manchester contest to capture the continental belt.
The venue became quiet for a extremely emotional memorial prior to the bout as a bell ceremony rang in memory of the city's native Hatton, who succumbed aged 46 in last month.
It was the premier televised match card in the location since Hatton's passing. As the final chime disappeared, the stillness gave way to a roar of "only one Hatton".
The competition within the ring equaled the emotion. The eventual winner was knocked down by a significant blow flush on the chin in the initial stage, standing uncertainly to his footing on the referee's count.
But the Manchester United-red equipment of the fighter soon advanced again. Across the ring, Rea - in competitive blue - was marked up quickly, heavy swelling forming under his face as the fighters traded powerful shots.
Rea, twenty-seven, delivered effective strikes but Arthur, thirty-four, matched the intensity with powerful upward punches as the intensity seldom decreased over 12 rounds.
The victor barely secured the verdict with scoring tallies of even score, narrow lead and 115-112, but each competitor were persistent and full of heart, all the qualities their hero Hatton stood for.
