British light-heavyweight Buatsi earned a win yet unimpressive beginning to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a split decision in a competitive 10-round bout at the boxing arena.
But the challenger landed significant shots, landing some telling single shots in a match where the techniques never quite clicked, with excessive holding spoiling the fight.
Two judges had it 96-94 for to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my greatest showing but what do champions do? They win," Buatsi commented in the arena as he was faced negative reactions from the spectators.
The Londoner got back to winning after his first professional defeat - an memorable fight with Callum Smith in February - to secure a 20th career victory.
Several boxing experts believed the challenger, thirty-one, merited the decision but the contender endured the second loss of his 28-match professional journey.
The Champion Secures Victory in Slow Encounter
His career development has stalled in recent times through contract problems and extended periods of inactivity.
Earlier forecast as the next star of the British fight scene, he now is positioned trying to achieve expectations - maybe in what seems like a ultimate opportunity - under recent support.
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 breaking his hand in the contest, was making just his second bout at the higher weight class.
By the middle rounds, Parker had kept it close, making life awkward for the favorite, who operated with a tight guard to barely take the more effective stanzas.
The spectators' acceptance decreased amid regular grabbing, with the challenger falling to the floor various instances during grappling.
The underdog finished the stronger, but many rounds were hard to judge. However, ex-champion Carl Frampton - commentating on Dazn - had it significantly in favor for the challenger, giving Buatsi just one round.
The victor moves on, but with the caliber of hard hitter David Benavidez, and Russian star Bivol leading the category, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Memorial Inspires Local Rivalry Battle
In the co-main event, Arthur overcame Rea by split-decision in a excellent local rivalry fight to secure the continental title.
The arena fell silent for a extremely emotional homage ahead of the contest as a bell ceremony rang in honor of Manchester's own Ricky Hatton, who succumbed 46 years old in September.
It was the first major televised fight night in the city since Hatton's passing. As the concluding sound disappeared, the quiet became a chant of "only one Ricky Hatton".
The action during the bout equaled the feeling. Arthur was floored by a significant blow flush on the face in the second round, getting up unsteadily to his footing on the eight count.
But the red-colored attire of the fighter soon advanced again. From the other corner, the opponent - in rival City blue - was marked up quickly, heavy swelling developing under his face as the pair landed hard punches.
The challenger, 27, connected with strong blows but the champion, thirty-four, responded in kind with powerful upward punches as the action hardly diminished over 12 rounds.
Arthur narrowly won the decision with judges scorecards of 114-114, 115-113 and wider gap, but each competitor were relentless and full of heart, everything their hero the legend stood for.
