British light-heavyweight Buatsi made a winning yet disappointing start to his career under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a tight ten-round fight at the Manchester venue.
But Parker landed significant shots, landing some telling individual punches in a match where the styles never quite clicked, with frequent clinching marring the action.
The scoring judges scored it 96-94 to the winner, with the third a even score.
"It wasn't my greatest showing but what defines champions? They succeed," the victor said in the arena as he was met by disapproval from the crowd.
The British fighter bounced back after his initial pro loss - an thrilling contest with Callum Smith in last winter - to secure a 20th career victory.
Various analysts felt the challenger, thirty-one, merited the win but the Derbyshire fighter suffered the second setback of his 28-bout career.
The Champion Claims Triumph in Dull Match
The fighter's advancement has plateaued in the past few years through contract problems and extended periods of inactivity.
Earlier forecast as the rising talent of British boxing, he now is positioned trying to achieve expectations - perhaps in what seems like a ultimate opportunity - under recent support.
He started carefully, with both competitors willing to overcommit in a tentative initial rounds.
The challenger, whose single earlier setback came at super middleweight against John Ryder in two years ago after damaging his hand during the bout, was competing in only his second fight at light heavyweight.
By the halfway stage, the underdog had stayed in contention, making life awkward for Buatsi, who worked behind a protective stance to edge the cleaner periods.
The audience's tolerance decreased amid frequent holding, with Parker slipping to the canvas multiple occasions during grappling.
The underdog finished the stronger, but several stanzas were difficult to score. Nevertheless, ex-champion the boxing analyst - working with Dazn - had it particularly wide for Parker, giving Buatsi just one round.
The victor advances, but with the likes of power puncher Benavidez, and Eastern European fighter Bivol ruling the division, the path to championship success - on this evidence - still looks steep.
Ricky Hatton Tribute Motivates All-Manchester Contest
In the supporting bout, Arthur beat Rea by close verdict in a outstanding all-Manchester contest to capture the European light-heavyweight belt.
The stadium grew hushed for a extremely emotional memorial before the fight as a bell ceremony tolled in memory of the city's native Hatton, who died at age 46 in September.
It was the initial significant broadcast fight night in the city since the passing of Hatton. As the last bell faded, the quiet became a chant of "only one Ricky Hatton".
The fighting inside the ropes equaled the emotion. Arthur was sent to the canvas by a significant blow directly to the face in the second round, rising shakily to his footing on the referee's count.
But the red-colored equipment of the fighter soon pressed ahead again. Across the ring, Rea - in competitive blue - was injured soon, significant bruising appearing under his face as the pair landed powerful shots.
The challenger, twenty-seven, landed power punches but Arthur, 34, matched the intensity with effective uppercuts as the action hardly diminished over the full distance.
The victor narrowly won the result with judges scorecards of even score, 115-113 and 115-112, but both fighters were determined and brave, everything their inspiration the legend stood for.
