UK light-heavyweight Buatsi made a winning yet underwhelming beginning to his career under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's punch output likely gained him a majority decision in a competitive ten-round bout at the boxing arena.
But Parker found success, landing some telling clean blows in a fight where the styles failed to mesh, with excessive holding spoiling the action.
The scoring judges had it 96-94 for to Buatsi, with the third a 95-95 draw.
"It wasn't my finest performance but what defines champions? They succeed," the victor said in the arena as he was faced negative reactions from the spectators.
The British fighter bounced back after his maiden career setback - an thrilling contest with his previous opponent in February - to earn a 20th career victory.
Some ringside observers believed the challenger, thirty-one, should have been awarded the win but the Derbyshire fighter suffered the second setback of his 28-match career.
Buatsi Claims Win in Uneventful Match
His career development has stalled in the past few years through promotional disputes and long spells of lack of fights.
Earlier forecast as the next star of the British fight scene, he now finds himself trying to realize his potential - possibly in what feels like a ultimate opportunity - under new promotional backing.
He started carefully, with neither fighter ready to take risks in a calculated initial rounds.
Parker, whose only previous defeat came at super-middleweight against John Ryder in 2022 after damaging his hand mid-fight, was making just his second fight at light heavyweight.
By the fight's midpoint, Parker had made it competitive, causing problems for Buatsi, who operated with a tight guard to narrowly win the more effective periods.
The spectators' acceptance decreased during frequent holding, with Parker falling to the floor several times during grappling.
Parker ended stronger, but multiple periods were challenging to assess. Nevertheless, ex-champion the boxing analyst - providing analysis for the broadcast network - had it particularly wide for the challenger, awarding Buatsi just a single round.
The victor advances, but with the presence of power puncher Benavidez, and Russian star Dmitry Bivol ruling the division, the road to world glory - based on this performance - still seems difficult.
Hatton Homage Fuels Local Rivalry Fight
In the supporting bout, Lyndon Arthur beat Rea by divided decision in a terrific local rivalry contest to secure the continental title.
The arena became quiet for a deeply poignant tribute before the fight as 10 bells sounded in remembrance of Manchester's own Hatton, who died aged 46 in September.
It was the first major televised boxing event in the location since Hatton's passing. As the concluding sound ended, the quiet gave way to a chant of "only one Ricky Hatton".
The action within the ring equaled the sentiment. Arthur was floored by a significant blow directly to the chin in the second round, rising shakily to his footing on the referee's count.
But the traditional red shorts of the fighter soon advanced again. Across the ring, Rea - in rival City blue - was marked up quickly, heavy swelling forming under his orbital area as the duo traded powerful shots.
Rea, 27, landed power punches but the champion, thirty-four, responded in kind with effective uppercuts as the pace rarely dropped over the full distance.
Arthur barely secured the verdict with scoring tallies of even score, close margin and 115-112, but each competitor were determined and brave, exactly what their inspiration Hatton embodied.
