UK light heavyweight Buatsi earned a win yet unimpressive debut to life under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a judges' verdict in a competitive 10-round contest at the Co-op Live Arena.
But the challenger had his moments, delivering effective individual punches in a contest where the techniques failed to mesh, with excessive holding spoiling the fight.
The officials scored it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my greatest performance but how do champions respond? They win," the victor stated in the ringside as he was greeted with negative reactions from the audience.
The British fighter got back to winning after his initial pro loss - an memorable fight with Callum Smith in last winter - to earn a 20th pro success.
Various analysts felt Parker, 31, should have been awarded the victory but the British boxer suffered the second setback of his 28-fight career.
Buatsi Claims Win in Uneventful Affair
Buatsi's progress has slowed in the past few years through contract problems and considerable time of lack of fights.
Previously predicted 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 ultimate opportunity - under new promotional backing.
He opened tentatively, with neither fighter willing to be aggressive in a calculated early stages.
Parker, whose sole prior loss came at the lower weight class against his former opponent in two years ago after breaking his hand in the contest, was participating in his second fight at light-heavyweight.
By the halfway stage, Parker had stayed in contention, creating difficulties for the favorite, who worked behind a protective stance to edge the more effective rounds.
The crowd's patience wore thin amid constant clinching, with the challenger going down several times during close exchanges.
Parker ended stronger, but several stanzas were challenging to assess. Nevertheless, previous titleholder Carl Frampton - working with the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just a single round.
Buatsi progresses, but with the caliber of heavy hitter David Benavidez, and Russian star Bivol dominating the weight class, the path to championship success - based on this performance - still appears challenging.
Ricky Hatton Homage Fuels All-Manchester Contest
In the secondary fight, Arthur defeated Rea by divided decision in a outstanding all-Manchester battle to win the continental belt.
The stadium fell silent for a profoundly moving homage ahead of the contest as ten bells tolled in memory of Manchester's own Ricky Hatton, who passed away aged 46 in September.
It was the initial significant broadcast boxing event in the urban center since Hatton's death. As the concluding sound ended, the quiet transitioned to a roar of "only one Ricky Hatton".
The competition within the ring matched the emotion. The eventual winner was floored by a left hook squarely on the face in the initial stage, standing uncertainly to his position on the eight count.
But the red-colored equipment of the fighter soon came forward again. From the other corner, Rea - in opposing blue - was injured soon, heavy swelling forming under his face as the fighters exchanged punishing blows.
Rea, 27, connected with strong blows but Arthur, 34, matched the intensity with powerful upward punches as the intensity seldom decreased over the full distance.
The victor edged the result with scoring tallies of even score, close margin and 115-112, but both fighters were persistent and courageous, everything their inspiration the legend stood for.
