UK light-heavyweight Buatsi earned a win yet unimpressive beginning to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a competitive 10-round contest at the Co-op Live Arena.
But Parker found success, connecting with powerful single shots in a fight where the approaches never quite clicked, with excessive holding disrupting the contest.
The scoring judges scored it 96-94 for to the winner, with the remaining official a 95-95 draw.
"It wasn't my greatest showing but how do champions respond? 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 epic battle with his previous opponent in February - to claim a twentieth professional win.
Some ringside observers thought the challenger, 31, should have been awarded the win but the contender experienced the second defeat of his 28-bout professional journey.
The Champion Earns Triumph in Slow Match
Buatsi's progress has stalled in the past few years through promotional disputes and long spells of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now stands trying to achieve expectations - possibly in what appears to be a ultimate opportunity - under fresh management.
He started carefully, with both competitors willing to take risks in a calculated opening few rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in two years ago after breaking his hand during the bout, was making just his second fight at the higher weight class.
By the middle rounds, Parker had kept it close, causing problems for Buatsi, who utilized a tight guard to edge the better stanzas.
The spectators' acceptance wore thin because of regular grabbing, with Parker falling to the floor various instances during close exchanges.
Parker finished the stronger, but many rounds were difficult to score. Although, former world champion Carl Frampton - commentating on Dazn - had it strongly leaning for Parker, scoring for Buatsi just a single round.
The victor progresses, but with the caliber of power puncher Benavidez, and Russian champion Bivol ruling the division, the road to world glory - on this evidence - still seems difficult.
Ricky Hatton Homage Inspires All-Manchester Battle
In the secondary fight, Lyndon Arthur beat Rea by close verdict in a excellent local rivalry battle to win the continental title.
The stadium grew hushed for a deeply poignant homage ahead of the contest as ten bells sounded in remembrance of Manchester's own Hatton, who died aged 46 in recently.
It was the initial significant broadcast fight night in the urban center since the passing of Hatton. As the final chime faded, the quiet transitioned to a chant of "only one Ricky Hatton".
The fighting during the bout reflected the emotion. The eventual winner was knocked down by a left hook directly to the chin in the initial stage, standing uncertainly to his footing on the referee's count.
But the traditional red attire of the fighter soon advanced again. From the other corner, the opponent - in opposing blue - was damaged quickly, heavy swelling developing under his orbital area as the pair landed punishing blows.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with powerful upward punches as the action seldom decreased over 12 rounds.
Arthur barely secured the result with scoring tallies of 114-114, 115-113 and 115-112, but both fighters were persistent and brave, exactly what their idol the legend represented.
