UK light-heavyweight Buatsi secured a victory yet disappointing start to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a split decision in a tight 10-round contest at the Manchester venue.
But Parker had his moments, connecting with powerful individual punches in a fight where the techniques failed to mesh, with constant grabbing disrupting the action.
The scoring judges judged it 96-94 for to Buatsi, with the final judge a even score.
"It wasn't my best performance but what defines champions? They win," the victor stated in the ring as he was faced boos from the spectators.
The Londoner bounced back after his first professional defeat - an thrilling contest with his previous opponent in February - to secure a 20th career victory.
Various analysts believed the challenger, 31, merited the victory but the contender suffered the second loss of his 28-fight professional journey.
The Champion Claims Win in Slow Encounter
His career development has plateaued in the past few years through management issues and considerable time of lack of fights.
Once tipped as the rising talent of UK boxing, he now stands trying to achieve expectations - perhaps in what seems like a last chance - under fresh management.
He began cautiously, with the two boxers prepared to take risks in a tentative early stages.
Parker, whose sole prior loss came at super-middleweight against John Ryder in two years ago after damaging his hand in the contest, was participating in his second fight at light-heavyweight.
By the middle rounds, Parker had made it competitive, creating difficulties for the favorite, who operated with a tight guard to edge the better rounds.
The spectators' acceptance decreased amid regular grabbing, with Parker slipping to the canvas various instances during grappling.
Parker finished the stronger, but several stanzas were challenging to assess. Nevertheless, previous titleholder Carl Frampton - commentating on Dazn - had it particularly wide for Parker, awarding Buatsi just a single round.
The victor moves on, but with the caliber of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the journey to titles - based on this performance - still looks steep.
Ricky Hatton Homage Fuels Manchester Derby Contest
In the supporting bout, Arthur defeated Rea by close verdict in a excellent local rivalry battle to secure the continental championship.
The arena fell silent for a profoundly moving tribute ahead of the contest as ten bells sounded in honor of the city's native Hatton, who passed away aged 46 in last month.
It was the initial significant broadcast match card in the city since Hatton's death. As the final chime disappeared, the stillness gave way to a roar of "there's only one Ricky Hatton".
The competition during the bout reflected the emotion. The eventual winner was knocked down by a powerful punch squarely on the jaw in the early round, getting up unsteadily to his position on the count of eight.
But the red-colored attire of the fighter soon came forward again. From the other corner, Rea - in competitive blue - was injured quickly, heavy swelling appearing under his face as the pair traded punishing blows.
Rea, 27, delivered effective strikes but Arthur, 34, responded in kind with effective uppercuts as the pace hardly diminished over twelve rounds.
Arthur narrowly won the result with judges scorecards of draw, narrow lead and wider gap, but both fighters were determined and full of heart, all the qualities their inspiration the legend represented.
