British light heavyweight Buatsi secured a victory yet underwhelming start to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a judges' verdict in a close 10-round bout at the Manchester venue.
But the challenger landed significant shots, landing some telling individual punches in a fight where the approaches didn't complement each other, with constant grabbing spoiling the fight.
The scoring judges had it 96-94 for to the winner, with the final judge a 95-95.
"It wasn't my greatest display but what defines champions? They win," the victor said in the arena as he was greeted with boos from the crowd.
The British fighter returned to winning ways after his initial pro loss - an thrilling contest with Callum Smith in February - to earn a 20th pro success.
Some ringside observers believed the challenger, 31, should have been awarded the win but the contender suffered the second defeat of his 28-match professional journey.
The Champion Claims Victory in Dull Encounter
The fighter's advancement has plateaued in recent times through promotional disputes and long spells of inactivity.
Previously predicted as the future of UK boxing, he now is positioned trying to achieve expectations - maybe in what seems like a ultimate opportunity - under recent support.
He opened tentatively, with neither fighter willing to take risks in a calculated early stages.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in two years ago after damaging his hand in the contest, was making just his second bout at light heavyweight.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for the favorite, who worked behind a protective stance to barely take the cleaner stanzas.
The spectators' acceptance wore thin because of regular grabbing, with Parker going down various instances during grappling.
The underdog closed well, but multiple periods were challenging to assess. Nevertheless, former world champion Carl Frampton - providing analysis for the broadcast network - had it particularly wide for the challenger, giving Buatsi just only one stanza.
Buatsi progresses, but with the likes of power puncher Benavidez, and Russian star Dmitry Bivol ruling the division, the path to championship success - based on this performance - still seems difficult.
Ricky Hatton Homage Fuels All-Manchester Battle
In the secondary fight, Arthur defeated Bradley Rea by close verdict in a terrific all-Manchester fight to win the continental belt.
The stadium became quiet for a profoundly moving homage before the fight as 10 bells tolled in remembrance of the city's native Ricky Hatton, who died at age 46 in September.
It was the initial significant broadcast boxing event in the location since Hatton's passing. As the final chime ended, the stillness became a chant of "there's just one Ricky Hatton".
The fighting within the ring reflected the sentiment. Arthur was knocked down by a left hook squarely on the jaw in the early round, getting up unsteadily to his position on the count of eight.
But the Manchester United-red shorts of the fighter soon advanced again. From the other corner, the opponent - in opposing blue - was injured quickly, heavy swelling forming under his orbital area as the pair exchanged hard punches.
The challenger, twenty-seven, landed power punches but Arthur, 34, fought fire with fire with effective uppercuts as the action seldom decreased over 12 rounds.
Arthur barely secured the decision with scoring tallies of 114-114, close margin and wider gap, but the two athletes were determined and brave, all the qualities their hero Hatton embodied.
