British light-heavyweight Buatsi secured a victory yet unimpressive beginning to his career under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a split decision in a competitive 10-round bout at the Manchester venue.
But the challenger landed significant shots, delivering effective single shots in a match where the styles didn't complement each other, with constant grabbing marring the action.
Two judges had it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my finest performance but how do champions respond? They succeed," Buatsi commented in the arena as he was met by disapproval from the audience.
The Londoner bounced back after his maiden career setback - an thrilling contest with his previous opponent in February - to claim a 20th pro success.
Several boxing experts believed Parker, 31, deserved the decision but the contender suffered the second defeat of his 28-bout career.
The Champion Earns Triumph in Slow Affair
His career development has slowed in recent years through contract problems and long spells of inactivity.
Previously predicted as the future of UK boxing, he now stands trying to achieve expectations - possibly in what appears to be a last chance - under new promotional backing.
He began cautiously, with the two boxers ready to overcommit in a cagey early stages.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in 2022 after breaking his hand in the contest, was participating in his second appearance at the higher weight class.
By the halfway stage, Parker had made it competitive, making life awkward for Buatsi, who utilized a solid defense to narrowly win the better periods.
The crowd's patience diminished during regular grabbing, with the challenger slipping to the canvas several times during close exchanges.
Parker finished the stronger, but several stanzas were challenging to assess. Nevertheless, former world champion the boxing analyst - providing analysis for Dazn - had it particularly wide for Parker, scoring for Buatsi just a single round.
The victor moves on, but with the caliber of hard hitter Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still seems difficult.
Hatton Tribute Inspires Local Rivalry Contest
In the supporting bout, Lyndon Arthur defeated Bradley Rea by divided decision in a terrific all-Manchester battle to win the European light heavyweight belt.
The arena fell silent for a deeply poignant memorial before the fight as 10 bells tolled in remembrance of the city's native Ricky Hatton, who died 46 years old in September.
It was the first major televised fight night in the urban center since Hatton's death. As the last bell disappeared, the quiet gave way to a roar of "only one Hatton".
The competition during the bout matched the feeling. The eventual winner was floored by a significant blow directly to the jaw in the second round, getting up unsteadily to his position on the referee's count.
But the red-colored shorts of Arthur soon pressed ahead again. From the other corner, Rea - in rival City blue - was damaged quickly, significant bruising developing under his eye as the fighters exchanged hard punches.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with powerful upward punches as the pace hardly diminished over the full distance.
The victor narrowly won the verdict with scoring tallies of draw, narrow lead and clearer margin, but each competitor were relentless and courageous, exactly what their inspiration the legend stood for.
