British light-heavyweight Buatsi made a winning yet unimpressive debut to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a close 10-round fight at the boxing arena.
But the challenger found success, landing some telling clean blows in a match where the styles failed to mesh, with constant grabbing spoiling the contest.
The officials had it 96-94 in favor to Buatsi, with the remaining official a 95-95.
"It wasn't my greatest performance but what do champions do? They prevail," the victor said in the ringside as he was greeted with disapproval from the audience.
The British fighter got back to winning after his initial pro loss - an memorable fight with Callum Smith in February - to earn a 20th pro success.
Several boxing experts felt the challenger, thirty-one, deserved the decision but the British boxer experienced the second loss of his 28-bout career.
The Champion Earns Victory in Slow Encounter
His career development has stalled in recent times through promotional disputes and long spells of inactivity.
Once tipped as the rising talent of British boxing, he now finds himself trying to fulfil that early promise - perhaps in what appears to be a last chance - under new promotional backing.
He started carefully, with neither fighter prepared to take risks in a cagey opening few rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in two years ago after breaking his hand during the bout, was making just his second bout at light-heavyweight.
By the fight's midpoint, Parker had made it competitive, making life awkward for the favorite, who worked behind a tight guard to barely take the better periods.
The audience's tolerance wore thin amid constant clinching, with Parker slipping to the canvas various instances during grappling.
Parker finished the stronger, but many rounds were hard to judge. However, former world champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for the challenger, giving Buatsi just a single round.
Buatsi moves on, but with the likes of hard hitter Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - on this evidence - still seems difficult.
Hatton Tribute Motivates Local Rivalry Fight
In the secondary fight, Arthur defeated Rea by divided decision in a outstanding local rivalry battle to win the European light-heavyweight belt.
The arena fell silent for a profoundly moving memorial before the fight as ten bells sounded in remembrance of Manchester's own Ricky Hatton, who succumbed 46 years old in last month.
It was the premier televised fight night in the city since Hatton's passing. As the last bell faded, the stillness transitioned to a roar of "there's only one Hatton".
The action during the bout equaled the sentiment. Arthur was knocked down by a powerful punch flush on the jaw in the initial stage, rising shakily to his feet on the eight count.
But the traditional red attire of Arthur soon advanced again. From the other corner, Rea - in opposing blue - was injured soon, noticeable damage appearing under his eye as the duo exchanged powerful shots.
The challenger, twenty-seven, landed power punches but the champion, 34, fought fire with fire with stinging uppercuts as the action rarely dropped over twelve rounds.
Arthur narrowly won the result with judges scorecards of 114-114, close margin and clearer margin, but each competitor were relentless and full of heart, everything their idol the legend stood for.
