UK light heavyweight Buatsi made a winning yet underwhelming beginning to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a majority decision in a close 10-round contest at the boxing arena.
But the challenger landed significant shots, landing some telling single shots in a contest where the styles didn't complement each other, with excessive holding disrupting the action.
The officials scored it 96-94 for to Buatsi, with the third a 95-95 draw.
"It wasn't my greatest display but what do champions do? They succeed," the victor stated in the arena as he was greeted with disapproval from the audience.
The Londoner returned to winning ways after his initial pro loss - an epic battle with his previous opponent in February - to earn a 20th career victory.
Various analysts felt the challenger, thirty-one, merited the win but the contender suffered the second loss of his 28-match career.
The Champion Claims Triumph in Slow Affair
The fighter's advancement has stalled in recent times through management issues and considerable time of inactivity.
Once tipped as the rising talent of UK boxing, he now is positioned trying to realize his potential - possibly in what appears to be a last chance - under recent support.
He began cautiously, with neither fighter willing to take risks in a calculated early stages.
Parker, whose only previous defeat came at super-middleweight against his former opponent in 2022 after breaking his hand in the contest, was making just his second appearance at the higher weight class.
By the fight's midpoint, Parker had kept it close, making life awkward for Buatsi, who utilized a protective stance to narrowly win the cleaner periods.
The spectators' acceptance decreased during constant clinching, with Parker slipping to the canvas various instances during grappling.
Parker ended stronger, but several stanzas were difficult to score. Although, former world champion Carl Frampton - working with the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just only one stanza.
Buatsi advances, but with the likes of heavy hitter David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still appears challenging.
Ricky Hatton Tribute Inspires Manchester Derby Fight
In the supporting bout, Lyndon Arthur beat Bradley Rea by split-decision in a outstanding all-Manchester fight to win the continental championship.
The venue became quiet for a profoundly moving homage ahead of the contest as 10 bells tolled in remembrance of the city's native Hatton, who died 46 years old in last month.
It was the initial significant broadcast boxing event in the location since the passing of Hatton. As the last bell faded, the stillness gave way to a roar of "only one Ricky Hatton".
The action inside the ropes reflected the feeling. The eventual winner was floored by a powerful punch flush on the face in the initial stage, getting up unsteadily to his feet on the eight count.
But the traditional red equipment of Arthur soon came forward again. Across the ring, the opponent - in competitive blue - was marked up quickly, heavy swelling appearing under his face as the duo landed punishing blows.
The challenger, 27, landed power punches but the champion, thirty-four, fought fire with fire with stinging uppercuts as the pace hardly diminished over the full distance.
The victor barely secured the decision with scoring tallies of draw, narrow lead and 115-112, but the two athletes were determined and courageous, everything their inspiration the legend represented.
