British light-heavyweight Buatsi secured a victory yet unimpressive start to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output seemingly secured him a judges' verdict in a close 10-round fight at the Co-op Live Arena.
But Parker had his moments, landing some telling clean blows in a contest where the styles failed to mesh, with frequent clinching marring the fight.
The officials had it 96-94 in favor to the winner, with the final judge a 95-95.
"It wasn't my finest display but what do champions do? They win," Buatsi commented in the arena as he was greeted with boos from the audience.
The British fighter returned to winning ways after his first professional defeat - an epic battle with his previous opponent in February - to earn a twentieth professional win.
Various analysts thought the challenger, 31, merited the decision but the contender endured the second loss of his 28-fight professional journey.
Buatsi Secures Victory in Uneventful Match
Buatsi's progress has slowed in recent times through management issues and extended periods of lack of fights.
Earlier forecast as the next star of UK boxing, he now finds himself trying to achieve expectations - possibly in what appears to be a last chance - under new promotional backing.
He began cautiously, with both competitors prepared to overcommit in a cagey early stages.
The challenger, whose only previous defeat came at super middleweight against his former opponent in two years ago after suffering a hand injury in the contest, was competing in only his second bout at the higher weight class.
By the fight's midpoint, Parker had kept it close, making life awkward for the favorite, who utilized a solid defense to edge the better stanzas.
The crowd's patience diminished amid regular grabbing, with the challenger slipping to the canvas multiple occasions during grappling.
The underdog ended stronger, but multiple periods were difficult to score. However, previous titleholder the boxing analyst - commentating on Dazn - had it strongly leaning for Parker, scoring for Buatsi just a single round.
Buatsi advances, but with the likes of power puncher Benavidez, and Eastern European fighter Bivol leading the category, the path to championship success - based on this performance - still seems difficult.
Ricky Hatton Tribute Inspires Local Rivalry Battle
In the supporting bout, Arthur beat Rea by divided decision in a terrific Manchester vs Manchester battle to secure the European light heavyweight championship.
The venue fell silent for a profoundly moving memorial ahead of the contest as ten bells sounded in memory of local hero Hatton, who died at age 46 in last month.
It was the premier televised boxing event in the location since Hatton's death. As the concluding sound faded, the quiet gave way to a chant of "there's only one Hatton".
The fighting within the ring equaled the sentiment. The eventual winner was sent to the canvas by a powerful punch directly to the face in the second round, getting up unsteadily to his position on the eight count.
But the red-colored shorts of Arthur soon pressed ahead again. On the opposite side, Rea - in competitive blue - was injured soon, heavy swelling forming under his eye as the pair exchanged powerful shots.
The challenger, 27, connected with strong blows but Arthur, 34, matched the intensity with powerful upward punches as the pace rarely dropped over 12 rounds.
The victor edged the decision with judges scorecards of 114-114, 115-113 and wider gap, but each competitor were persistent and full of heart, all the qualities their inspiration Hatton represented.
