UK light-heavyweight Buatsi secured a victory yet unimpressive start to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a close ten-round fight at the boxing arena.
But the challenger landed significant shots, connecting with powerful individual punches in a contest where the approaches failed to mesh, with excessive holding disrupting the fight.
The scoring judges judged it 96-94 for to the winner, with the remaining official a 95-95 draw.
"It wasn't my best showing but how do champions respond? They succeed," the victor said in the ring as he was faced disapproval from the crowd.
The British fighter got back to winning after his maiden career setback - an thrilling contest with Callum Smith in February - to claim a 20th pro success.
Various analysts felt the challenger, thirty-one, should have been awarded the decision but the contender experienced the second setback of his 28-match career.
Buatsi Claims Victory in Slow Affair
His career development has plateaued in recent years through promotional disputes and extended periods of inactivity.
Earlier forecast as the rising talent of British boxing, he now stands trying to realize his potential - possibly in what seems like a last chance - under new promotional backing.
He started carefully, with neither fighter willing to take risks in a cagey early stages.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in 2022 after breaking his hand mid-fight, was making just his second fight at light-heavyweight.
By the middle rounds, Parker had kept it close, creating difficulties for the favorite, who operated with a protective stance to edge the cleaner periods.
The crowd's patience decreased amid regular grabbing, with the challenger falling to the floor multiple occasions during clinches.
Parker finished the stronger, but many rounds were challenging to assess. However, ex-champion the boxing analyst - providing analysis for Dazn - had it significantly in favor for the challenger, scoring for Buatsi just only one stanza.
The victor progresses, but with the likes of power puncher Benavidez, and Russian star Bivol dominating the weight class, the path to championship success - judging by this showing - still appears challenging.
Hatton Memorial Fuels Local Rivalry Fight
In the supporting bout, Arthur defeated Rea by divided decision in a terrific all-Manchester contest to capture the European light-heavyweight title.
The venue fell silent for a deeply poignant homage prior to the bout as ten bells sounded in honor of Manchester's own Hatton, who passed away at age 46 in recently.
It was the first major televised match card in the urban center since the passing of Hatton. As the final chime ended, the quiet became a chant of "there's only one Ricky Hatton".
The fighting during the bout reflected the feeling. Arthur was knocked down by a left hook squarely on the chin in the early round, rising shakily to his feet on the eight count.
But the Manchester United-red equipment of the fighter soon advanced again. From the other corner, Rea - in competitive blue - was injured soon, significant bruising developing under his orbital area as the fighters landed powerful shots.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with powerful upward punches as the intensity rarely dropped over the full distance.
The victor narrowly won the decision with scoring tallies of 114-114, narrow lead and wider gap, but both fighters were persistent and brave, everything their inspiration Hatton represented.
