British light-heavyweight Buatsi made a winning yet underwhelming debut to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level seemingly secured him a split decision in a close ten-round bout at the boxing arena.
But the challenger had his moments, landing some telling individual punches in a contest where the techniques failed to mesh, with excessive holding disrupting the contest.
The officials had it 96-94 in favor to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my finest display but what do champions do? They succeed," Buatsi stated in the ring as he was greeted with negative reactions from the spectators.
The British fighter bounced back after his maiden career setback - an thrilling contest with Callum Smith in last winter - to claim a twentieth professional win.
Some ringside observers believed the challenger, thirty-one, deserved the victory but the contender endured the second defeat of his 28-match professional journey.
The Champion Earns Win in Slow Affair
Buatsi's progress has plateaued in recent times through management issues and considerable time of lack of fights.
Earlier forecast as the next star of British boxing, he now finds himself trying to realize his potential - maybe in what appears to be a final roll of the dice - under fresh management.
He opened tentatively, with neither fighter willing to overcommit in a cagey initial rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in 2022 after damaging his hand in the contest, was making just his second fight at light heavyweight.
By the halfway stage, the underdog had kept it close, creating difficulties for Buatsi, who operated with a protective stance to narrowly win the better rounds.
The audience's tolerance diminished during regular grabbing, with the challenger slipping to the canvas several times during close exchanges.
Parker ended stronger, but multiple periods were difficult to score. Although, previous titleholder the boxing analyst - commentating on the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just a single round.
Buatsi advances, but with the caliber of heavy hitter Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - on this evidence - still seems difficult.
Hatton Homage Motivates All-Manchester Contest
In the secondary fight, Arthur defeated Rea by split-decision in a terrific all-Manchester contest to capture the European light heavyweight championship.
The arena became quiet for a profoundly moving tribute prior to the bout as 10 bells tolled in honor of local hero Hatton, who died aged 46 in September.
It was the initial significant broadcast boxing event in the location since the passing of Hatton. As the concluding sound faded, the quiet gave way to a chant of "there's just one Hatton".
The competition within the ring matched the feeling. Arthur was floored by a significant blow flush on the chin in the second round, rising shakily to his position on the eight count.
But the Manchester United-red attire of the fighter soon pressed ahead again. From the other corner, the opponent - in opposing blue - was damaged soon, noticeable damage developing under his orbital area as the duo landed hard punches.
Rea, 27, landed power punches but the champion, 34, responded in kind with powerful upward punches as the action hardly diminished over twelve rounds.
The victor edged the verdict with judges scorecards of draw, close margin and wider gap, but the two athletes were determined and full of heart, exactly what their inspiration the legend stood for.
