UK light-heavyweight Buatsi earned a win yet disappointing debut to his career under new promoters with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's work rate seemingly secured him a judges' verdict in a close ten-round fight at the Manchester venue.
But Parker landed significant shots, delivering effective individual punches in a fight where the styles never quite clicked, with constant grabbing marring the action.
Two judges had it 96-94 in favor to the winner, with the remaining official a 95-95 draw.
"It wasn't my best showing but what do champions do? They win," the victor commented in the ring as he was met by disapproval from the audience.
The British fighter bounced back after his initial pro loss - an thrilling contest with his previous opponent in February - to secure a 20th pro success.
Several boxing experts thought the challenger, thirty-one, merited the win but the British boxer suffered the second loss of his 28-fight career.
The Champion Earns Victory in Uneventful Encounter
Buatsi's progress has plateaued in recent times through management issues and long spells of inactivity.
Previously predicted as the next star of UK boxing, he now stands trying to fulfil that early promise - possibly in what appears to be a final roll of the dice - under new promotional backing.
He started carefully, with both competitors willing to take risks in a calculated initial rounds.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in two years ago after damaging his hand during the bout, was making just his second bout at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for the favorite, who worked behind a tight guard to narrowly win the better periods.
The spectators' acceptance wore thin during constant clinching, with the challenger slipping to the canvas several times during clinches.
Parker finished the stronger, but many rounds were challenging to assess. Nevertheless, previous titleholder Carl Frampton - providing analysis for Dazn - had it particularly wide for the challenger, scoring for Buatsi just a single round.
Buatsi progresses, but with the caliber of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the path to championship success - on this evidence - still looks steep.
Ricky Hatton Homage Fuels Local Rivalry Battle
In the co-main event, Lyndon Arthur defeated Bradley Rea by divided decision in a terrific local rivalry contest to secure the European light-heavyweight championship.
The venue became quiet for a extremely emotional homage ahead of the contest as 10 bells tolled in memory of the city's native Ricky Hatton, who died 46 years old in last month.
It was the first major televised fight night in the city since Hatton's passing. As the last bell disappeared, the silence became a shout of "there's only one Hatton".
The competition within the ring reflected the feeling. Arthur was sent to the canvas by a left hook squarely on the face in the second round, rising shakily to his feet on the referee's count.
But the red-colored shorts of Arthur soon came forward again. Across the ring, the opponent - in rival City blue - was damaged soon, significant bruising appearing under his orbital area as the fighters landed hard punches.
The challenger, twenty-seven, landed power punches but Arthur, 34, matched the intensity with stinging uppercuts as the intensity hardly diminished over twelve rounds.
Arthur narrowly won the result with official scores of even score, narrow lead and 115-112, but both fighters were determined and courageous, exactly what their idol Hatton represented.
