British light heavyweight Buatsi secured a victory yet underwhelming start to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a judges' verdict in a competitive ten-round fight at the boxing arena.
But Parker had his moments, delivering effective individual punches in a match where the approaches failed to mesh, with frequent clinching disrupting the fight.
The scoring judges had it 96-94 for to Buatsi, with the final judge a 95-95 draw.
"It wasn't my greatest performance but what defines champions? They win," Buatsi said in the ringside as he was met by negative reactions from the audience.
The British fighter got back to winning after his initial pro loss - an memorable fight with Callum Smith in last winter - to claim a 20th pro success.
Various analysts thought the challenger, 31, merited the victory but the contender experienced the second defeat of his 28-fight professional journey.
Buatsi Earns Victory in Slow Encounter
Buatsi's progress has slowed in the past few years through promotional disputes and extended periods of inactivity.
Once tipped as the future of UK boxing, he now is positioned trying to achieve expectations - perhaps in what seems like a final roll of the dice - under new promotional backing.
He started carefully, with both competitors willing to be aggressive in a calculated initial rounds.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in two years ago after damaging his hand mid-fight, was making just his second fight at light heavyweight.
By the fight's midpoint, the underdog had kept it close, making life awkward for Buatsi, who utilized a tight guard to narrowly win the cleaner rounds.
The crowd's patience wore thin because of constant clinching, with the challenger slipping to the canvas multiple occasions during grappling.
Parker ended stronger, but many rounds were difficult to score. However, ex-champion Carl Frampton - commentating on the broadcast network - had it particularly wide for the challenger, giving Buatsi just one round.
Buatsi moves on, but with the caliber of power puncher David Benavidez, and Russian star Bivol leading the category, the road to world glory - on this evidence - still looks steep.
Hatton Memorial Fuels All-Manchester Battle
In the secondary fight, Arthur overcame Rea by split-decision in a outstanding Manchester vs Manchester contest to secure the European light-heavyweight title.
The stadium became quiet for a deeply poignant homage before the fight as 10 bells rang in honor of local hero Hatton, who passed away aged 46 in last month.
It was the premier televised match card in the city since Hatton's death. As the last bell ended, the quiet gave way to a roar of "there's just one Ricky Hatton".
The action within the ring reflected the sentiment. The eventual winner was sent to the canvas by a significant blow directly to the face in the second round, standing uncertainly to his footing on the eight count.
But the Manchester United-red attire of the fighter soon pressed ahead again. Across the ring, the opponent - in competitive blue - was damaged soon, significant bruising developing under his eye as the duo landed hard punches.
Rea, twenty-seven, landed power punches but the champion, thirty-four, responded in kind with effective uppercuts as the intensity rarely dropped over 12 rounds.
The victor barely secured the decision with scoring tallies of draw, 115-113 and wider gap, but each competitor were persistent and brave, everything their inspiration Hatton stood for.
