British light-heavyweight Buatsi secured a victory yet unimpressive beginning to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a majority decision in a close ten-round fight at the Co-op Live Arena.
But the challenger found success, delivering effective individual punches in a contest where the styles failed to mesh, with frequent clinching disrupting the fight.
The officials had it 96-94 for to the winner, with the remaining official a 95-95 draw.
"It wasn't my best showing but what do champions do? They win," Buatsi commented in the arena as he was greeted with boos from the spectators.
The Londoner returned to winning ways after his maiden career setback - an epic battle with Callum Smith in last winter - to earn a 20th career victory.
Some ringside observers felt the challenger, 31, merited the decision but the British boxer endured the second defeat of his 28-match career.
Buatsi Secures Win in Slow Encounter
Buatsi's progress has stalled in recent times through contract problems and long spells of inactivity.
Previously predicted as the next star of the British fight scene, he now stands trying to achieve expectations - maybe in what feels like a ultimate opportunity - under fresh management.
He started carefully, with both competitors prepared to take risks in a tentative initial rounds.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in 2022 after breaking his hand in the contest, was competing in only his second bout at the higher weight class.
By the halfway stage, Parker had kept it close, creating difficulties for Buatsi, who operated with a protective stance to narrowly win the better periods.
The spectators' acceptance decreased during frequent holding, with the challenger falling to the floor various instances during close exchanges.
Parker finished the stronger, but several stanzas were hard to judge. Nevertheless, ex-champion the boxing analyst - working with the broadcast network - had it particularly wide for Parker, awarding Buatsi just a single round.
Buatsi advances, but with the likes of heavy hitter Benavidez, and Russian champion Bivol dominating the weight class, the journey to titles - based on this performance - still looks steep.
Ricky Hatton Tribute Fuels Local Rivalry Contest
In the co-main event, Lyndon Arthur overcame Rea by close verdict in a excellent Manchester vs Manchester fight to capture the European light heavyweight belt.
The venue grew hushed for a deeply poignant tribute before the fight as 10 bells tolled in honor of the city's native Ricky Hatton, who died aged 46 in September.
It was the premier televised match card in the location since Hatton's death. As the final chime faded, the quiet transitioned to a chant of "there's only one Ricky Hatton".
The action within the ring reflected the emotion. Arthur was knocked down by a significant blow flush on the jaw in the second round, getting up unsteadily to his position on the eight count.
But the traditional red shorts of the fighter soon came forward again. On the opposite side, Rea - in competitive blue - was damaged soon, noticeable damage appearing under his orbital area as the fighters landed hard punches.
Rea, 27, landed power punches but the champion, 34, matched the intensity with powerful upward punches as the action rarely dropped over the full distance.
The victor narrowly won the decision with official scores of draw, narrow lead and wider gap, but each competitor were determined and full of heart, all the qualities their hero the legend embodied.
