UK light-heavyweight Buatsi earned a win yet unimpressive debut to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a competitive 10-round bout at the Co-op Live Arena.
But Parker had his moments, connecting with powerful single shots in a fight where the approaches didn't complement each other, with constant grabbing disrupting the contest.
Two judges scored it 96-94 for to the winner, with the third a 95-95.
"It wasn't my finest showing but what do champions do? They prevail," the victor said in the ring as he was greeted with disapproval from the spectators.
The British fighter got back to winning after his maiden career setback - an epic battle with Callum Smith in February - to earn a 20th pro success.
Several boxing experts felt the challenger, 31, merited the decision but the contender suffered the second defeat of his 28-match professional journey.
The Champion Claims Win in Dull Encounter
His career development has stalled in recent years through contract problems and long spells of inactivity.
Once tipped as the rising talent of UK boxing, he now finds himself trying to realize his potential - maybe in what appears to be a final roll of the dice - under fresh management.
He began cautiously, with the two boxers prepared to overcommit in a cagey early stages.
Parker, whose only previous defeat came at super-middleweight against his former opponent in two years ago after damaging his hand in the contest, was participating in his second bout at the higher weight class.
By the fight's midpoint, Parker had made it competitive, causing problems for the favorite, who utilized a protective stance to narrowly win the cleaner periods.
The audience's tolerance diminished because of constant clinching, with the challenger going down multiple occasions during clinches.
Parker finished the stronger, but many rounds were difficult to score. Nevertheless, ex-champion the boxing analyst - commentating on the broadcast network - had it particularly wide for the challenger, scoring for Buatsi just only one stanza.
Buatsi moves on, but with the caliber of heavy hitter David Benavidez, and Russian star Dmitry Bivol leading the category, the journey to titles - based on this performance - still looks steep.
Hatton Memorial Motivates Local Rivalry Contest
In the secondary fight, Arthur beat Rea by split-decision in a excellent local rivalry contest to win the European light-heavyweight belt.
The arena fell silent for a extremely emotional memorial prior to the bout as ten bells rang in remembrance of the city's native Hatton, who died 46 years old in last month.
It was the first major televised boxing event in the city since the passing of Hatton. As the concluding sound ended, the stillness transitioned to a roar of "there's just one Hatton".
The competition within the ring matched the emotion. Arthur was sent to the canvas by a powerful punch squarely on the face in the second round, rising shakily to his footing on the count of eight.
But the red-colored equipment of Arthur soon advanced again. Across the ring, the opponent - in rival City blue - was damaged quickly, significant bruising developing under his face as the duo landed powerful shots.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, responded in kind with stinging uppercuts as the intensity seldom decreased over twelve rounds.
Arthur barely secured the decision with official scores of draw, 115-113 and 115-112, but the two athletes were determined and full of heart, everything their hero the legend embodied.
