British light-heavyweight Buatsi secured a victory yet underwhelming debut to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a competitive 10-round fight at the Manchester venue.
But Parker landed significant shots, connecting with powerful single shots in a fight where the techniques never quite clicked, with constant grabbing spoiling the contest.
The scoring judges scored it 96-94 for to Buatsi, with the final judge a 95-95.
"It wasn't my best display but what do champions do? They succeed," the victor stated in the ringside as he was greeted with negative reactions from the audience.
The Londoner got back to winning after his first professional defeat - an memorable fight with Callum Smith in last winter - to earn a twentieth professional win.
Various analysts felt Parker, 31, deserved the win but the British boxer suffered the second loss of his 28-fight career.
Buatsi Secures Triumph in Dull Encounter
Buatsi's progress has plateaued in the past few years through contract problems and long spells of lack of fights.
Previously predicted as the rising talent of UK boxing, he now finds himself trying to achieve expectations - maybe in what appears to be a last chance - under new promotional backing.
He opened tentatively, with the two boxers willing to take risks in a cagey initial rounds.
Parker, whose single earlier setback came at super middleweight against his former opponent in two years ago after suffering a hand injury mid-fight, was participating in his second bout at the higher weight class.
By the fight's midpoint, the underdog had made it competitive, causing problems for the favorite, who operated with a protective stance to narrowly win the cleaner periods.
The crowd's patience decreased during frequent holding, with the challenger falling to the floor several times during grappling.
The underdog ended stronger, but many rounds were difficult to score. Nevertheless, former world champion the boxing analyst - commentating on the broadcast network - had it particularly wide for the challenger, awarding Buatsi just only one stanza.
The victor progresses, 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 appears challenging.
Hatton Tribute Motivates Manchester Derby Battle
In the secondary fight, Arthur defeated Rea by close verdict in a terrific local rivalry contest to secure the continental title.
The arena became quiet for a extremely emotional memorial prior to the bout as ten bells rang in honor of the city's native Hatton, who passed away aged 46 in September.
It was the initial significant broadcast fight night in the location since Hatton's death. As the concluding sound ended, the silence became a roar of "there's only one Hatton".
The competition within the ring reflected the sentiment. The eventual winner was sent to the canvas by a powerful punch squarely on the face in the second round, rising shakily to his footing on the referee's count.
But the traditional red equipment of Arthur soon came forward again. From the other corner, the opponent - in opposing blue - was damaged soon, noticeable damage appearing under his face as the fighters traded hard punches.
Rea, twenty-seven, delivered effective strikes but Arthur, thirty-four, matched the intensity with powerful upward punches as the pace seldom decreased over twelve rounds.
The victor narrowly won the verdict with scoring tallies of 114-114, close margin and wider gap, but both fighters were determined and brave, exactly what their inspiration the legend stood for.
