British light-heavyweight Buatsi secured a victory yet disappointing beginning to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate seemingly secured him a majority decision in a tight 10-round fight at the Co-op Live Arena.
But Parker had his moments, landing some telling individual punches in a fight where the styles never quite clicked, with excessive holding spoiling the contest.
Two judges had it 96-94 in favor to the winner, with the third a 95-95.
"It wasn't my best showing but how do champions respond? They win," the victor said in the arena as he was greeted with disapproval from the audience.
The Londoner got back to winning after his initial pro loss - an thrilling contest with his previous opponent in February - to claim a 20th career victory.
Various analysts thought Parker, 31, should have been awarded the victory but the Derbyshire fighter experienced the second setback of his 28-match professional journey.
Buatsi Claims Victory in Uneventful Match
The fighter's advancement has slowed in the past few years through contract problems and considerable time of inactivity.
Once tipped as the next star of the British fight scene, he now is positioned trying to realize his potential - perhaps in what seems like a final roll of the dice - under fresh management.
He began cautiously, with both competitors prepared to be aggressive in a cagey initial rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after suffering a hand injury in the contest, was competing in only his second bout at light-heavyweight.
By the middle rounds, the underdog had kept it close, creating difficulties for the favorite, who utilized a solid defense to barely take the more effective periods.
The crowd's patience diminished because of frequent holding, with Parker falling to the floor several times during close exchanges.
Parker ended stronger, but multiple periods were difficult to score. However, previous titleholder Carl Frampton - providing analysis for Dazn - had it significantly in favor for the challenger, giving Buatsi just one round.
Buatsi advances, but with the likes of power puncher Benavidez, and Eastern European fighter Bivol dominating the weight class, the road to world glory - based on this performance - still looks steep.
Hatton Homage Inspires Local Rivalry Battle
In the co-main event, Arthur beat Bradley Rea by divided decision in a excellent Manchester vs Manchester battle to secure the continental belt.
The venue fell silent for a profoundly moving memorial before the fight as ten bells tolled in memory of the city's native Hatton, who died at age 46 in September.
It was the initial significant broadcast match card in the location since the passing of Hatton. As the final chime disappeared, the quiet transitioned to a shout of "there's just one Ricky Hatton".
The competition inside the ropes equaled the sentiment. Arthur was sent to the canvas by a significant blow squarely on the chin in the second round, standing uncertainly to his footing on the count of eight.
But the traditional red equipment of the fighter soon advanced again. From the other corner, the opponent - in rival City blue - was marked up quickly, heavy swelling forming under his orbital area as the duo exchanged hard punches.
The challenger, 27, landed power punches but the champion, 34, matched the intensity with effective uppercuts as the intensity hardly diminished over the full distance.
Arthur narrowly won the verdict with scoring tallies of even score, close margin and wider gap, but the two athletes were persistent and courageous, exactly what their hero the legend stood for.
