British light-heavyweight Buatsi secured a victory yet unimpressive beginning to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's activity level seemingly secured him a split decision in a close ten-round contest at the Manchester venue.
But the challenger landed significant shots, delivering effective individual punches in a match where the styles didn't complement each other, with excessive holding spoiling the fight.
Two judges judged it 96-94 for to Buatsi, with the final judge a even score.
"It wasn't my finest performance but what defines champions? They prevail," the victor commented in the ring as he was greeted with boos from the spectators.
The Londoner got back to winning after his initial pro loss - an memorable fight with Callum Smith in February - to secure a 20th pro success.
Several boxing experts thought Parker, thirty-one, merited the victory but the Derbyshire fighter endured the second loss of his 28-bout professional journey.
The Champion Claims Win in Uneventful Encounter
Buatsi's progress has plateaued in the past few years through management issues and long spells of inactivity.
Previously predicted as the next star of the British fight scene, he now stands trying to fulfil that early promise - maybe in what seems like a final roll of the dice - under new promotional backing.
He opened tentatively, with both competitors prepared to be aggressive in a cagey initial rounds.
Parker, whose single earlier setback came at the lower weight class against his former opponent in two years ago after damaging his hand during the bout, was making just his second bout at light heavyweight.
By the middle rounds, the underdog had stayed in contention, making life awkward for the favorite, who operated with a solid defense to narrowly win the better rounds.
The crowd's patience diminished because of frequent holding, with the challenger falling to the floor several times during grappling.
The underdog finished the stronger, but multiple periods were challenging to assess. However, former world champion Carl Frampton - providing analysis for the broadcast network - had it particularly wide for the challenger, scoring for Buatsi just one round.
Buatsi moves on, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Bivol leading the category, the path to championship success - on this evidence - still seems difficult.
Hatton Memorial Fuels All-Manchester Battle
In the supporting bout, Lyndon Arthur beat Rea by close verdict in a excellent local rivalry contest to capture the continental belt.
The arena grew hushed for a profoundly moving tribute ahead of the contest as 10 bells sounded in memory of the city's native Hatton, who passed away aged 46 in September.
It was the first major televised match card in the city since the passing of Hatton. As the final chime ended, the quiet became a shout of "only one Ricky Hatton".
The competition during the bout reflected the emotion. The eventual winner was sent to the canvas by a significant blow flush on the face in the initial stage, standing uncertainly to his footing on the referee's count.
But the Manchester United-red equipment of Arthur soon pressed ahead again. From the other corner, the opponent - in opposing blue - was marked up quickly, noticeable damage forming under his eye as the pair exchanged hard punches.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, responded in kind with stinging uppercuts as the intensity seldom decreased over the full distance.
Arthur narrowly won the decision with judges scorecards of draw, close margin and 115-112, but each competitor were determined and full of heart, all the qualities their idol Hatton embodied.
