UK light heavyweight Buatsi earned a win yet disappointing start to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a judges' verdict in a competitive ten-round contest at the boxing arena.
But Parker found success, landing some telling clean blows in a match where the styles failed to mesh, with excessive holding marring the contest.
The scoring judges had it 96-94 in favor to the winner, with the third a 95-95.
"It wasn't my best performance but what do champions do? They succeed," the victor commented in the ring as he was met by boos from the spectators.
The Londoner bounced back after his maiden career setback - an memorable fight with his previous opponent in February - to earn a 20th career victory.
Several boxing experts thought the challenger, 31, should have been awarded the win but the Derbyshire fighter experienced the second defeat of his 28-fight professional journey.
The Champion Claims Victory in Dull Match
The fighter's advancement has plateaued in recent years through promotional disputes and long spells of inactivity.
Earlier forecast as the future of British boxing, he now is positioned trying to realize his potential - possibly in what feels like a final roll of the dice - under new promotional backing.
He opened tentatively, with both competitors prepared to take risks in a tentative opening few rounds.
Parker, whose single earlier setback came at the lower weight class against John Ryder in two years ago after suffering a hand injury during the bout, was competing in only his second bout at the higher weight class.
By the fight's midpoint, the underdog had kept it close, creating difficulties for the favorite, who operated with a solid defense to edge the more effective periods.
The crowd's patience diminished amid regular grabbing, with Parker slipping to the canvas multiple occasions during clinches.
The underdog closed well, but multiple periods were challenging to assess. Nevertheless, ex-champion the boxing analyst - working with the broadcast network - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
The victor moves on, but with the likes of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the path to championship success - on this evidence - still looks steep.
Hatton Tribute Inspires Local Rivalry Battle
In the secondary fight, Lyndon Arthur beat Rea by close verdict in a excellent Manchester vs Manchester battle to capture the European light heavyweight title.
The venue became quiet for a profoundly moving homage before the fight as a bell ceremony sounded in memory of Manchester's own Hatton, who died at age 46 in September.
It was the initial significant broadcast match card in the city since Hatton's passing. As the concluding sound ended, the stillness gave way to a shout of "there's only one Hatton".
The competition within the ring reflected the feeling. Arthur was knocked down by a powerful punch flush on the face in the initial stage, standing uncertainly to his position on the eight count.
But the red-colored shorts of the fighter soon advanced again. Across the ring, the opponent - in opposing blue - was marked up quickly, noticeable damage forming under his face as the fighters exchanged powerful shots.
Rea, twenty-seven, landed power punches but Arthur, 34, fought fire with fire with effective uppercuts as the intensity seldom decreased over 12 rounds.
The victor barely secured the decision with scoring tallies of even score, narrow lead and 115-112, but both fighters were relentless and full of heart, exactly what their inspiration the legend represented.
