British light heavyweight Buatsi earned a win yet disappointing debut to his career under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a judges' verdict in a tight ten-round bout at the Manchester venue.
But the challenger found success, connecting with powerful individual punches in a match where the approaches didn't complement each other, with excessive holding disrupting the action.
Two judges had it 96-94 for to Buatsi, with the final judge a 95-95.
"It wasn't my greatest display but how do champions respond? They prevail," the victor commented in the ring as he was faced disapproval from the crowd.
The Londoner got back to winning after his first professional defeat - an memorable fight with Callum Smith in last winter - to secure a 20th career victory.
Several boxing experts believed Parker, thirty-one, deserved the victory but the British boxer experienced the second defeat of his 28-match professional journey.
The Champion Secures Win in Dull Affair
His career development has plateaued in recent times through contract problems and long spells of inactivity.
Once tipped as the next star of the British fight scene, he now stands trying to fulfil that early promise - maybe in what seems like a last chance - under fresh management.
He began cautiously, with neither fighter willing to be aggressive in a cagey opening few rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in two years ago after breaking his hand mid-fight, was making just his second fight at light heavyweight.
By the halfway stage, Parker had made it competitive, causing problems for Buatsi, who operated with a protective stance to narrowly win the cleaner periods.
The spectators' acceptance decreased during constant clinching, with the challenger slipping to the canvas various instances during clinches.
The underdog finished the stronger, but many rounds were difficult to score. Nevertheless, previous titleholder Carl Frampton - commentating on Dazn - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
Buatsi progresses, but with the caliber of heavy hitter Benavidez, and Russian star Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still seems difficult.
Hatton Tribute Motivates All-Manchester Battle
In the supporting bout, Lyndon Arthur beat Rea by divided decision in a outstanding all-Manchester contest to capture the European light-heavyweight belt.
The venue became quiet for a extremely emotional homage prior to the bout as 10 bells sounded in honor of Manchester's own Ricky Hatton, who succumbed 46 years old in recently.
It was the initial significant broadcast match card in the urban center since Hatton's passing. As the final chime disappeared, the quiet became a chant of "only one Hatton".
The competition inside the ropes equaled the sentiment. The eventual winner was floored by a significant blow flush on the chin in the second round, standing uncertainly to his feet on the count of eight.
But the red-colored attire of Arthur soon pressed ahead again. On the opposite side, the opponent - in rival City blue - was damaged quickly, significant bruising forming under his face as the fighters traded punishing blows.
Rea, twenty-seven, landed power punches but Arthur, thirty-four, fought fire with fire with powerful upward punches as the action hardly diminished over 12 rounds.
The victor edged the result with official scores of 114-114, close margin and clearer margin, but both fighters were persistent and full of heart, exactly what their inspiration the legend represented.
