UK light heavyweight Buatsi made a winning 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 ten-round contest at the boxing arena.
But Parker landed significant shots, landing some telling clean blows in a fight where the techniques failed to mesh, with frequent clinching spoiling the action.
Two judges scored it 96-94 for to the winner, with the third a even score.
"It wasn't my finest performance but what defines champions? They prevail," Buatsi commented in the arena as he was faced negative reactions from the crowd.
The Londoner got back to winning after his maiden career setback - an memorable fight with his previous opponent in last winter - to earn a 20th pro success.
Various analysts believed Parker, thirty-one, merited the victory but the Derbyshire fighter experienced the second defeat of his 28-match career.
The Champion Earns Win in Slow Match
His career development has slowed in recent times through promotional disputes and long spells of lack of fights.
Once tipped as the next star of British boxing, he now stands trying to fulfil that early promise - perhaps in what feels like a ultimate opportunity - under new promotional backing.
He began cautiously, with neither fighter ready to take risks in a tentative opening few rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in two years ago after damaging his hand in the contest, was making just his second appearance at light heavyweight.
By the middle rounds, the underdog had made it competitive, creating difficulties for the favorite, who worked behind a tight guard to edge the cleaner rounds.
The audience's tolerance diminished because of constant clinching, with Parker going down several times during close exchanges.
Parker closed well, but many rounds were hard to judge. However, previous titleholder the boxing analyst - working with Dazn - had it strongly leaning for the challenger, awarding Buatsi just only one stanza.
The victor advances, but with the caliber of hard hitter Benavidez, and Eastern European fighter Bivol dominating the weight class, the path to championship success - on this evidence - still seems difficult.
Hatton Homage Motivates Manchester Derby Fight
In the supporting bout, Lyndon Arthur defeated Rea by split-decision in a terrific all-Manchester contest to capture the European light-heavyweight title.
The stadium grew hushed for a profoundly moving memorial before the fight as a bell ceremony tolled in memory of local hero Ricky Hatton, who succumbed aged 46 in recently.
It was the initial significant broadcast match card in the location since the passing of Hatton. As the concluding sound disappeared, the quiet gave way to a shout of "there's just one Hatton".
The fighting during the bout matched the feeling. The eventual winner was sent to the canvas by a significant blow directly to the face in the early round, standing uncertainly to his position on the count of eight.
But the traditional red equipment of Arthur soon advanced again. Across the ring, Rea - in opposing blue - was marked up quickly, heavy swelling appearing under his orbital area as the duo landed powerful shots.
The challenger, 27, connected with strong blows but the champion, 34, fought fire with fire with effective uppercuts as the pace hardly diminished over twelve rounds.
The victor barely secured the decision with judges scorecards of even score, 115-113 and wider gap, but both fighters were persistent and full of heart, exactly what their hero Hatton embodied.
