British light-heavyweight Buatsi earned a win yet unimpressive start to life under new promoters with a decision win over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a majority decision in a close 10-round bout at the Co-op Live Arena.
But Parker had his moments, delivering effective single shots in a fight where the styles didn't complement each other, with excessive holding spoiling the contest.
Two judges had it 96-94 to Buatsi, with the final judge a 95-95.
"It wasn't my finest performance but what do champions do? They win," the victor said in the ring as he was greeted with disapproval from the spectators.
The Londoner bounced back after his maiden career setback - an thrilling contest with Callum Smith in last winter - to earn a 20th pro success.
Some ringside observers felt the challenger, thirty-one, merited the victory but the British boxer suffered the second setback of his 28-bout career.
Buatsi Earns Victory in Dull Affair
The fighter's advancement has plateaued in the past few years through promotional disputes and long spells of inactivity.
Earlier forecast as the next star of UK boxing, he now finds himself trying to realize his potential - maybe in what seems like a ultimate opportunity - under fresh management.
He began cautiously, with the two boxers prepared to take risks in a cagey early stages.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in two years ago after damaging his hand in the contest, was making just his second bout at light-heavyweight.
By the middle rounds, Parker had stayed in contention, making life awkward for Buatsi, who utilized a solid defense to narrowly win the better stanzas.
The spectators' acceptance diminished during constant clinching, with the challenger falling to the floor various instances during close exchanges.
Parker ended stronger, but multiple periods were difficult to score. However, former world champion Carl Frampton - working with Dazn - had it particularly wide for the challenger, giving Buatsi just one round.
The victor advances, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the path to championship success - judging by this showing - still appears challenging.
Ricky Hatton Tribute Motivates All-Manchester Battle
In the secondary fight, Arthur overcame Rea by divided decision in a terrific all-Manchester fight to win the continental championship.
The stadium became quiet for a profoundly moving tribute ahead of the contest as ten bells sounded in honor of Manchester's own Hatton, who died 46 years old in recently.
It was the first major televised fight night in the city since the passing of Hatton. As the concluding sound faded, the stillness gave way to a roar of "only one Hatton".
The action inside the ropes reflected the emotion. The eventual winner was knocked down by a powerful punch directly to the chin in the initial stage, standing uncertainly to his feet on the count of eight.
But the traditional red attire of the fighter soon came forward again. On the opposite side, Rea - in opposing blue - was injured quickly, noticeable damage appearing under his orbital area as the duo landed punishing blows.
Rea, 27, connected with strong blows but the champion, 34, fought fire with fire with stinging uppercuts as the pace rarely dropped over twelve rounds.
The victor edged the verdict with judges scorecards of 114-114, close margin and 115-112, but each competitor were persistent and courageous, all the qualities their idol the legend represented.
