British light heavyweight Buatsi earned a win yet underwhelming start to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's activity level seemingly secured him a split decision in a tight ten-round fight at the boxing arena.
But Parker found success, delivering effective single shots in a fight where the techniques failed to mesh, with constant grabbing disrupting the action.
The scoring judges had it 96-94 for to Buatsi, with the third a 95-95 draw.
"It wasn't my greatest display but what defines champions? They succeed," Buatsi stated in the ringside as he was met by negative reactions from the audience.
The British fighter bounced back after his maiden career setback - an memorable fight with his previous opponent in last winter - to earn a twentieth professional win.
Some ringside observers thought the challenger, thirty-one, merited the win but the British boxer suffered the second setback of his 28-bout professional journey.
The Champion Earns Win in Dull Affair
His career development has plateaued in the past few years through promotional disputes and considerable time of lack of fights.
Earlier forecast as the next star of the British fight scene, he now finds himself trying to fulfil that early promise - possibly in what feels like a final roll of the dice - under recent support.
He opened tentatively, with the two boxers prepared to overcommit in a cagey early stages.
Parker, whose only previous defeat came at super-middleweight against his former opponent in two years ago after damaging his hand in the contest, was participating in his second appearance at the higher weight class.
By the halfway stage, Parker had kept it close, creating difficulties for Buatsi, who utilized a protective stance to narrowly win the better stanzas.
The spectators' acceptance diminished because of regular grabbing, with the challenger slipping to the canvas multiple occasions during clinches.
The underdog finished the stronger, but many rounds were difficult to score. Nevertheless, previous titleholder Carl Frampton - commentating on the broadcast network - had it significantly in favor for the challenger, giving Buatsi just one round.
Buatsi progresses, but with the caliber of power puncher David Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - judging by this showing - still looks steep.
Hatton Homage Motivates Local Rivalry Fight
In the secondary fight, Arthur defeated Rea by split-decision in a outstanding Manchester vs Manchester contest to capture the continental belt.
The arena fell silent for a profoundly moving tribute before the fight as 10 bells sounded in remembrance of Manchester's own Ricky Hatton, who succumbed aged 46 in September.
It was the first major televised fight night in the urban center since Hatton's death. As the final chime ended, the stillness became a chant of "there's only one Ricky Hatton".
The action during the bout equaled the sentiment. Arthur was knocked down by a significant blow directly to the jaw in the early round, getting up unsteadily to his footing on the count of eight.
But the Manchester United-red attire of Arthur soon pressed ahead again. From the other corner, Rea - in rival City blue - was marked up quickly, heavy swelling forming under his eye as the duo traded punishing blows.
Rea, 27, landed power punches but the champion, thirty-four, responded in kind with effective uppercuts as the pace seldom decreased over 12 rounds.
Arthur narrowly won the verdict with scoring tallies of draw, 115-113 and 115-112, but the two athletes were persistent and full of heart, everything their inspiration Hatton stood for.
