British light-heavyweight Buatsi secured a victory yet unimpressive start to his career under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output seemingly secured him a judges' verdict in a tight ten-round fight at the Manchester venue.
But Parker found success, connecting with powerful individual punches in a fight where the approaches failed to mesh, with constant grabbing disrupting the action.
Two judges scored it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my best showing but how do champions respond? They succeed," the victor commented in the ringside as he was greeted with boos from the spectators.
The Londoner got back to winning after his first professional defeat - an thrilling contest with his previous opponent in last winter - to claim a 20th pro success.
Some ringside observers believed the challenger, 31, should have been awarded the victory but the Derbyshire fighter endured the second loss of his 28-match career.
The Champion Secures Victory in Slow Match
Buatsi's progress has slowed in the past few years through promotional disputes and extended periods of inactivity.
Earlier forecast as the next star of UK boxing, he now stands trying to achieve expectations - possibly in what appears to be a final roll of the dice - under fresh management.
He opened tentatively, with both competitors ready to overcommit in a cagey early stages.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was participating in his second bout at the higher weight class.
By the fight's midpoint, Parker had kept it close, creating difficulties for the favorite, who operated with a solid defense to edge the better rounds.
The audience's tolerance decreased amid regular grabbing, with the challenger going down multiple occasions during clinches.
The underdog ended stronger, but multiple periods were difficult to score. However, ex-champion the boxing analyst - commentating on the broadcast network - had it particularly wide for Parker, scoring for Buatsi just a single round.
Buatsi progresses, but with the likes of hard hitter Benavidez, and Russian star Bivol ruling the division, the road to world glory - based on this performance - still seems difficult.
Ricky Hatton Homage Fuels All-Manchester Contest
In the co-main event, Arthur defeated Rea by split-decision in a outstanding local rivalry fight to capture the European light-heavyweight belt.
The venue became quiet for a extremely emotional tribute before the fight as 10 bells sounded in memory of Manchester's own Hatton, who succumbed aged 46 in last month.
It was the first major televised fight night in the location since Hatton's passing. As the final chime faded, the quiet gave way to a chant of "only one Hatton".
The action within the ring equaled the sentiment. Arthur was sent to the canvas by a significant blow directly to the jaw in the second round, rising shakily to his footing on the eight count.
But the traditional red equipment of Arthur soon pressed ahead again. On the opposite side, the opponent - in opposing blue - was injured quickly, heavy swelling developing under his eye as the duo traded hard punches.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, responded in kind with stinging uppercuts as the intensity seldom decreased over the full distance.
The victor barely secured the decision with scoring tallies of 114-114, close margin and 115-112, but both fighters were persistent and full of heart, exactly what their idol Hatton stood for.
