UK light-heavyweight Buatsi earned a win yet disappointing debut to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a majority decision in a close ten-round bout at the Manchester venue.
But the challenger landed significant shots, landing some telling single shots in a match where the styles never quite clicked, with excessive holding marring the contest.
Two judges judged it 96-94 for to Buatsi, with the final judge a 95-95.
"It wasn't my finest display but what defines champions? They succeed," Buatsi said in the ring as he was met by disapproval from the crowd.
The Londoner bounced back after his first professional defeat - an thrilling contest with his previous opponent in February - to earn a 20th pro success.
Some ringside observers believed the challenger, thirty-one, deserved the victory but the British boxer suffered the second defeat of his 28-match career.
The Champion Earns Triumph in Dull Match
His career development has stalled in recent years through promotional disputes and considerable time of inactivity.
Previously predicted as the next star of UK boxing, he now finds himself trying to achieve expectations - possibly in what appears to be a final roll of the dice - under recent support.
He opened tentatively, with both competitors ready to take risks in a calculated early stages.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after suffering a hand injury during the bout, was participating in his second appearance at light-heavyweight.
By the middle rounds, the underdog had kept it close, causing problems for Buatsi, who utilized a tight guard to narrowly win the cleaner periods.
The crowd's patience wore thin because of regular grabbing, with the challenger going down multiple occasions during close exchanges.
Parker ended stronger, but multiple periods were hard to judge. Although, ex-champion Carl Frampton - commentating on Dazn - had it strongly leaning for Parker, giving Buatsi just only one stanza.
Buatsi moves on, but with the presence of heavy hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still appears challenging.
Hatton Memorial Motivates Manchester Derby Battle
In the supporting bout, Lyndon Arthur beat Rea by divided decision in a excellent local rivalry contest to capture the European light-heavyweight title.
The venue fell silent for a extremely emotional tribute prior to the bout as ten bells rang in honor of local hero Ricky Hatton, who passed away aged 46 in last month.
It was the initial significant broadcast match card in the city since Hatton's death. As the final chime faded, the silence became a chant of "there's just one Hatton".
The fighting during the bout reflected the emotion. Arthur was floored by a powerful punch directly to the face in the early round, standing uncertainly to his footing on the eight count.
But the red-colored attire of Arthur soon pressed ahead again. On the opposite side, Rea - in competitive blue - was marked up early, noticeable damage developing under his face as the pair traded hard punches.
The challenger, 27, delivered effective strikes but the champion, thirty-four, matched the intensity with effective uppercuts as the pace rarely dropped over the full distance.
Arthur edged the verdict with scoring tallies of draw, 115-113 and 115-112, but each competitor were determined and full of heart, exactly what their hero Hatton embodied.
