British light-heavyweight Buatsi secured a victory yet disappointing beginning to his career under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a judges' verdict in a competitive ten-round bout at the Manchester venue.
But Parker had his moments, landing some telling clean blows in a contest where the styles never quite clicked, with frequent clinching spoiling the action.
The scoring judges judged it 96-94 for to the winner, with the final judge a 95-95.
"It wasn't my best display but what defines champions? They succeed," the victor commented in the arena as he was faced negative reactions from the spectators.
The Londoner bounced back after his initial pro loss - an thrilling contest with his previous opponent in February - to claim a twentieth professional win.
Some ringside observers thought Parker, thirty-one, deserved the decision but the British boxer suffered the second setback of his 28-match career.
Buatsi Secures Victory in Dull Match
His career development has slowed in recent years through management issues and long spells of lack of fights.
Once tipped as the next star of UK boxing, he now is positioned trying to fulfil that early promise - possibly in what feels like a ultimate opportunity - under new promotional backing.
He began cautiously, with neither fighter ready to be aggressive in a calculated initial rounds.
Parker, whose single earlier setback came at the lower weight class against John Ryder in two years ago after breaking his hand in the contest, was participating in his second bout at light heavyweight.
By the halfway stage, the underdog had kept it close, creating difficulties for Buatsi, who operated with a solid defense to barely take the better periods.
The spectators' acceptance decreased because of frequent holding, with the challenger falling to the floor multiple occasions during clinches.
Parker closed well, but many rounds were difficult to score. Nevertheless, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it significantly in favor for Parker, awarding Buatsi just one round.
The victor advances, but with the presence of heavy hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the path to championship success - based on this performance - still seems difficult.
Ricky Hatton Memorial Motivates Manchester Derby Fight
In the supporting bout, Lyndon Arthur beat Rea by split-decision in a excellent all-Manchester contest to secure the continental championship.
The venue fell silent for a extremely emotional tribute before the fight as ten bells sounded in memory of Manchester's own Hatton, who succumbed at age 46 in September.
It was the premier televised fight night in the urban center since Hatton's passing. As the final chime faded, the stillness transitioned to a roar of "only one Ricky Hatton".
The competition within the ring matched the feeling. The eventual winner was floored by a left hook flush on the jaw in the early round, getting up unsteadily to his position on the referee's count.
But the red-colored equipment of Arthur soon advanced again. From the other corner, the opponent - in competitive blue - was damaged early, noticeable damage forming under his face as the duo landed punishing blows.
Rea, 27, connected with strong blows but Arthur, thirty-four, matched the intensity with powerful upward punches as the intensity hardly diminished over twelve rounds.
Arthur narrowly won the decision with judges scorecards of even score, close margin and 115-112, but both fighters were determined and courageous, all the qualities their inspiration the legend represented.
