UK light-heavyweight Buatsi made a winning yet unimpressive start to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a majority decision in a tight ten-round bout at the Manchester venue.
But Parker found success, landing some telling individual punches in a fight where the styles failed to mesh, with constant grabbing spoiling the fight.
The officials scored it 96-94 for to Buatsi, with the remaining official a even score.
"It wasn't my greatest performance but what do champions do? They succeed," Buatsi said in the ring as he was met by boos from the audience.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with his previous opponent in last winter - to claim a 20th pro success.
Some ringside observers felt Parker, thirty-one, deserved the win but the contender experienced the second defeat of his 28-fight career.
Buatsi Claims Win in Dull Affair
Buatsi's progress has plateaued in recent times through management issues and extended periods of inactivity.
Previously predicted as the future of British boxing, he now is positioned trying to achieve expectations - perhaps in what appears to be a last chance - under fresh management.
He began cautiously, with both competitors ready to overcommit in a cagey early stages.
Parker, whose single earlier setback came at the lower weight class against his former opponent in two years ago after damaging his hand during the bout, was competing in only his second fight at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, making life awkward for the favorite, who operated with a tight guard to narrowly win the better stanzas.
The crowd's patience decreased during regular grabbing, with Parker falling to the floor various instances during close exchanges.
Parker ended stronger, but multiple periods were difficult to score. Nevertheless, ex-champion Carl Frampton - commentating on Dazn - had it significantly in favor for Parker, awarding Buatsi just a single round.
Buatsi advances, but with the likes of power puncher David Benavidez, and Russian champion Bivol leading the category, the journey to titles - on this evidence - still seems difficult.
Hatton Memorial Inspires Manchester Derby Contest
In the supporting bout, Lyndon Arthur beat Rea by split-decision in a terrific local rivalry battle to secure the European light heavyweight championship.
The stadium fell silent for a extremely emotional memorial before the fight as 10 bells rang in honor of the city's native Hatton, who passed away aged 46 in September.
It was the initial significant broadcast fight night in the location since Hatton's death. As the final chime faded, the stillness transitioned to a shout of "only one Ricky Hatton".
The action inside the ropes equaled the feeling. The eventual winner was knocked down by a left hook squarely on the jaw in the early round, standing uncertainly to his footing on the count of eight.
But the Manchester United-red shorts of the fighter soon came forward again. Across the ring, Rea - in rival City blue - was damaged soon, significant bruising developing under his eye as the fighters exchanged hard punches.
Rea, twenty-seven, landed power punches but the champion, thirty-four, matched the intensity with powerful upward punches as the pace rarely dropped over the full distance.
Arthur edged the result with scoring tallies of draw, close margin and wider gap, but the two athletes were persistent and full of heart, everything their hero the legend stood for.
