UK light heavyweight Buatsi made a winning yet underwhelming beginning to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a split decision in a close 10-round contest at the boxing arena.
But Parker found success, connecting with powerful clean blows in a fight where the styles failed to mesh, with frequent clinching marring the contest.
Two judges had it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my best display but what do champions do? They prevail," Buatsi stated in the ring as he was greeted with disapproval from the spectators.
The British fighter bounced back after his maiden career setback - an thrilling contest with his previous opponent in last winter - to secure a 20th pro success.
Some ringside observers felt the challenger, 31, merited the win but the British boxer experienced the second setback of his 28-bout career.
Buatsi Earns Triumph in Dull Affair
The fighter's advancement has slowed in the past few years through promotional disputes and long spells of inactivity.
Previously predicted as the next star of UK boxing, he now is positioned trying to achieve expectations - perhaps in what feels like a ultimate opportunity - under new promotional backing.
He began cautiously, with neither fighter ready to take risks in a cagey early stages.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in 2022 after damaging his hand mid-fight, was competing in only his second appearance at the higher weight class.
By the fight's midpoint, the underdog had made it competitive, causing problems for the favorite, who worked behind a protective stance to narrowly win the better periods.
The audience's tolerance diminished during regular grabbing, with Parker going down several times during clinches.
Parker ended stronger, but several stanzas were difficult to score. However, ex-champion Carl Frampton - working with Dazn - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
The victor moves on, but with the presence of power puncher Benavidez, and Russian star Dmitry Bivol dominating the weight class, the journey to titles - based on this performance - still looks steep.
Ricky Hatton Tribute Inspires All-Manchester Contest
In the supporting bout, Lyndon Arthur beat Rea by split-decision in a terrific all-Manchester fight to capture the continental championship.
The venue grew hushed for a profoundly moving homage before the fight as a bell ceremony sounded in honor of local hero Hatton, who succumbed 46 years old in September.
It was the premier televised match card in the city since the passing of Hatton. As the concluding sound faded, the silence became a roar of "only one Ricky Hatton".
The action during the bout equaled the sentiment. The eventual winner was sent to the canvas by a left hook flush on the jaw in the early round, standing uncertainly to his feet on the eight count.
But the Manchester United-red equipment of Arthur soon came forward again. Across the ring, the opponent - in opposing blue - was marked up early, heavy swelling forming under his orbital area as the duo exchanged punishing blows.
Rea, twenty-seven, landed power punches but the champion, thirty-four, matched the intensity with stinging uppercuts as the intensity rarely dropped over twelve rounds.
Arthur barely secured the verdict with scoring tallies of 114-114, close margin and 115-112, but the two athletes were persistent and full of heart, all the qualities their hero the legend represented.
