UK light-heavyweight Buatsi secured a victory yet underwhelming beginning to life under new promoters with a decision win over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a split decision in a tight 10-round bout at the Co-op Live Arena.
But the challenger found success, connecting with powerful individual punches in a match where the approaches didn't complement each other, with excessive holding disrupting the contest.
The scoring judges judged it 96-94 in favor to Buatsi, with the final judge a 95-95 draw.
"It wasn't my greatest display but what defines champions? They prevail," the victor stated in the ringside as he was faced disapproval from the spectators.
The British fighter returned to winning ways after his maiden career setback - an memorable fight with his previous opponent in February - to secure a 20th career victory.
Some ringside observers believed Parker, thirty-one, deserved the decision but the contender suffered the second defeat of his 28-fight career.
The Champion Earns Victory in Slow Affair
The fighter's advancement has plateaued in the past few years through contract problems and long spells of inactivity.
Earlier forecast as the future of British boxing, he now is positioned trying to realize his potential - maybe in what appears to be a last chance - under fresh management.
He started carefully, with the two boxers ready to overcommit in a cagey early stages.
The challenger, whose sole prior loss came at super middleweight against John Ryder in 2022 after suffering a hand injury mid-fight, was participating in his second fight at the higher weight class.
By the fight's midpoint, the underdog had stayed in contention, causing problems for Buatsi, who operated with a tight guard to edge the more effective rounds.
The crowd's patience wore thin because of frequent holding, with Parker slipping to the canvas various instances during close exchanges.
The underdog finished the stronger, but many rounds were hard to judge. Although, ex-champion Carl Frampton - commentating on Dazn - had it particularly wide for the challenger, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the likes of hard hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the road to world glory - on this evidence - still seems difficult.
Hatton Homage Inspires Manchester Derby Contest
In the co-main event, Lyndon Arthur beat Rea by close verdict in a excellent all-Manchester contest to secure the European light heavyweight belt.
The arena became quiet for a profoundly moving tribute ahead of the contest as 10 bells sounded in remembrance of local hero Hatton, who passed away 46 years old in September.
It was the first major televised match card in the urban center since Hatton's passing. As the concluding sound ended, the silence gave way to a chant of "only one Hatton".
The competition inside the ropes matched the feeling. The eventual winner was floored by a powerful punch directly to the jaw in the second round, rising shakily to his feet on the eight count.
But the traditional red equipment of Arthur soon advanced again. On the opposite side, the opponent - in competitive blue - was injured soon, noticeable damage forming under his orbital area as the pair exchanged powerful shots.
Rea, twenty-seven, delivered effective strikes but Arthur, thirty-four, fought fire with fire with powerful upward punches as the pace seldom decreased over twelve rounds.
The victor edged the verdict with official scores of draw, 115-113 and 115-112, but the two athletes were persistent and courageous, everything their idol Hatton represented.
