British light-heavyweight Buatsi secured a victory yet disappointing start to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a majority decision in a close 10-round contest at the boxing arena.
But the challenger found success, connecting with powerful individual punches in a fight where the approaches never quite clicked, with constant grabbing marring the contest.
The officials judged it 96-94 to the winner, with the remaining official a even score.
"It wasn't my best performance but what defines champions? They win," the victor commented in the ringside as he was faced boos from the crowd.
The Londoner got back to winning after his maiden career setback - an thrilling contest with Callum Smith in February - to claim a 20th career victory.
Some ringside observers thought Parker, thirty-one, should have been awarded the win but the Derbyshire fighter experienced the second setback of his 28-match professional journey.
Buatsi Earns Win in Dull Encounter
His career development has slowed in recent times through contract problems and considerable time of inactivity.
Once tipped as the rising talent of the British fight scene, he now finds himself trying to realize his potential - maybe in what feels like a final roll of the dice - under new promotional backing.
He started carefully, with neither fighter prepared to be aggressive in a cagey initial rounds.
Parker, whose single earlier setback came at super middleweight against John Ryder in two years ago after suffering a hand injury in the contest, was participating in his second fight at light-heavyweight.
By the middle rounds, Parker had made it competitive, causing problems for the favorite, who worked behind a solid defense to barely take the better periods.
The spectators' acceptance decreased during constant clinching, with Parker slipping to the canvas several times during grappling.
The underdog ended stronger, but several stanzas were hard to judge. Nevertheless, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for Parker, awarding Buatsi just one round.
The victor advances, but with the presence of hard hitter David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still looks steep.
Ricky Hatton Homage Inspires Manchester Derby Fight
In the supporting bout, Arthur overcame Rea by split-decision in a outstanding Manchester vs Manchester contest to secure the European light-heavyweight championship.
The arena grew hushed for a extremely emotional memorial before the fight as a bell ceremony tolled in remembrance of Manchester's own Ricky Hatton, who succumbed 46 years old in last month.
It was the premier televised boxing event in the city since Hatton's passing. As the concluding sound ended, the silence transitioned to a shout of "only one Hatton".
The action during the bout equaled the feeling. The eventual winner was floored by a significant blow flush on the chin in the early round, getting up unsteadily to his feet on the eight count.
But the traditional red shorts of Arthur soon pressed ahead again. On the opposite side, the opponent - in rival City blue - was damaged quickly, noticeable damage forming under his eye as the fighters landed punishing blows.
The challenger, 27, delivered effective strikes but Arthur, 34, matched the intensity with stinging uppercuts as the pace rarely dropped over the full distance.
The victor barely secured the decision with judges scorecards of 114-114, narrow lead and clearer margin, but both fighters were determined and courageous, everything their idol Hatton embodied.
