UK light-heavyweight Buatsi made a winning yet underwhelming beginning to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a split decision in a tight ten-round fight at the boxing arena.
But Parker found success, delivering effective clean blows in a match where the styles never quite clicked, with frequent clinching spoiling the fight.
The officials had it 96-94 for to Buatsi, with the final judge a 95-95 draw.
"It wasn't my best display but how do champions respond? They prevail," the victor stated in the ring as he was greeted with disapproval from the crowd.
The Londoner got back to winning after his first professional defeat - an memorable fight with Callum Smith in February - to secure a 20th pro success.
Various analysts believed the challenger, thirty-one, merited the win but the Derbyshire fighter suffered the second defeat of his 28-fight career.
Buatsi Secures Victory in Slow Match
Buatsi's progress has slowed in recent times through management issues and extended periods of lack of fights.
Once tipped as the future of the British fight scene, he now finds himself trying to achieve expectations - perhaps in what appears to be a last chance - under recent support.
He opened tentatively, with the two boxers ready to take risks in a tentative initial rounds.
Parker, whose only previous defeat came at super-middleweight against his former opponent in 2022 after suffering a hand injury in the contest, was competing in only his second fight at light heavyweight.
By the fight's midpoint, Parker had kept it close, creating difficulties for Buatsi, who utilized a solid defense to edge the better rounds.
The spectators' acceptance wore thin because of frequent holding, with Parker falling to the floor various instances during close exchanges.
The underdog finished the stronger, but several stanzas were hard to judge. Although, previous titleholder Carl Frampton - working with Dazn - had it particularly wide for Parker, scoring for Buatsi just a single round.
The victor moves on, but with the likes of power puncher Benavidez, and Russian star Bivol leading the category, the path to championship success - judging by this showing - still looks steep.
Hatton Tribute Motivates Local Rivalry Battle
In the secondary fight, Lyndon Arthur defeated Bradley Rea by divided decision in a excellent Manchester vs Manchester battle to capture the European light-heavyweight championship.
The venue became quiet for a extremely emotional tribute ahead of the contest as a bell ceremony rang in remembrance of local hero Hatton, who passed away at age 46 in September.
It was the premier televised match card in the city since Hatton's death. As the concluding sound disappeared, the stillness became a roar of "there's just one Ricky Hatton".
The competition within the ring reflected the emotion. Arthur was sent to the canvas by a powerful punch directly to the jaw in the second round, getting up unsteadily to his feet on the count of eight.
But the Manchester United-red shorts of the fighter soon came forward again. From the other corner, the opponent - in opposing blue - was damaged early, heavy swelling forming under his orbital area as the pair exchanged hard punches.
Rea, twenty-seven, landed power punches but the champion, thirty-four, matched the intensity with effective uppercuts as the intensity hardly diminished over twelve rounds.
Arthur narrowly won the decision with scoring tallies of 114-114, 115-113 and 115-112, but the two athletes were relentless and courageous, exactly what their idol the legend stood for.
