UK light-heavyweight Buatsi earned a win yet disappointing start to his career under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a majority decision in a competitive 10-round fight at the boxing arena.
But Parker landed significant shots, delivering effective clean blows in a fight where the approaches didn't complement each other, with frequent clinching disrupting the fight.
Two judges scored it 96-94 for to Buatsi, with the third a even score.
"It wasn't my finest display but what do champions do? They succeed," Buatsi stated in the arena as he was greeted with disapproval from the spectators.
The Londoner got back to winning after his maiden career setback - an thrilling contest with Callum Smith in last winter - to claim a 20th career victory.
Various analysts felt the challenger, 31, merited the win but the British boxer suffered the second defeat of his 28-fight career.
Buatsi Earns Triumph in Slow Affair
Buatsi's progress has slowed in recent years through contract problems and extended periods of lack of fights.
Once tipped as the next star of the British fight scene, he now stands trying to achieve expectations - maybe in what appears to be a last chance - under fresh management.
He started carefully, with both competitors prepared to take risks in a calculated opening few rounds.
Parker, whose single earlier setback 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 middle rounds, the underdog had made it competitive, creating difficulties for the favorite, who utilized a tight guard to narrowly win the better rounds.
The crowd's patience diminished because of constant clinching, with Parker falling to the floor several times during close exchanges.
The underdog closed well, but multiple periods were difficult to score. However, former world champion the boxing analyst - working with Dazn - had it strongly leaning for Parker, scoring for Buatsi just one round.
The victor moves on, but with the caliber of heavy hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the journey to titles - on this evidence - still looks steep.
Ricky Hatton Homage Motivates All-Manchester Battle
In the supporting bout, Lyndon Arthur defeated Rea by divided decision in a outstanding Manchester vs Manchester battle to win the European light heavyweight title.
The stadium fell silent for a profoundly moving tribute before the fight as ten bells rang in honor of Manchester's own Ricky Hatton, who passed away at age 46 in recently.
It was the initial significant broadcast boxing event in the location since the passing of Hatton. As the concluding sound faded, the silence gave way to a roar of "there's only one Ricky Hatton".
The fighting during the bout matched the emotion. Arthur was sent to the canvas by a powerful punch squarely on the face in the initial stage, rising shakily to his position on the referee's count.
But the traditional red attire of Arthur soon advanced again. From the other corner, the opponent - in opposing blue - was damaged soon, heavy swelling appearing under his face as the duo exchanged powerful shots.
Rea, twenty-seven, delivered effective strikes but the champion, 34, responded in kind with stinging uppercuts as the action rarely dropped over the full distance.
Arthur edged the verdict with judges scorecards of 114-114, 115-113 and wider gap, but the two athletes were relentless and brave, exactly what their hero the legend embodied.
