British light heavyweight Buatsi secured a victory yet disappointing beginning to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a judges' verdict in a tight ten-round contest at the Co-op Live Arena.
But the challenger found success, connecting with powerful single shots in a contest where the techniques failed to mesh, with excessive holding disrupting the contest.
The scoring judges scored it 96-94 to Buatsi, with the third a even score.
"It wasn't my best showing but how do champions respond? They prevail," the victor stated in the ring as he was met by disapproval from the spectators.
The British fighter bounced back after his initial pro loss - an memorable fight with Callum Smith in February - to secure a 20th pro success.
Various analysts felt the challenger, 31, should have been awarded the decision but the contender experienced the second defeat of his 28-bout professional journey.
Buatsi Claims Win in Slow Affair
The fighter's advancement has stalled in the past few years through promotional disputes and considerable time of lack of fights.
Previously predicted as the rising talent of British boxing, he now is positioned trying to realize his potential - maybe in what seems like a final roll of the dice - under recent support.
He began cautiously, with neither fighter ready to overcommit in a calculated initial rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in two years ago after suffering a hand injury mid-fight, was competing in only his second appearance at the higher weight class.
By the fight's midpoint, the underdog had made it competitive, causing problems for the favorite, who worked behind a tight guard to narrowly win the better stanzas.
The audience's tolerance diminished because of constant clinching, with Parker falling to the floor multiple occasions during clinches.
Parker closed well, but multiple periods were difficult to score. Although, ex-champion the boxing analyst - commentating on Dazn - had it particularly wide for Parker, scoring for Buatsi just one round.
The victor progresses, but with the likes of hard hitter David Benavidez, and Russian champion Bivol ruling the division, the path to championship success - based on this performance - still appears challenging.
Hatton Homage Inspires All-Manchester Battle
In the supporting bout, Lyndon Arthur beat Bradley Rea by close verdict in a excellent Manchester vs Manchester battle to capture the European light heavyweight title.
The stadium fell silent for a deeply poignant homage ahead of the contest as 10 bells sounded in memory of the city's native Ricky Hatton, who passed away 46 years old in last month.
It was the first major televised boxing event in the location since Hatton's passing. As the final chime faded, the stillness gave way to a roar of "only one Ricky Hatton".
The fighting during the bout matched the sentiment. The eventual winner was sent to the canvas by a significant blow flush on the chin in the initial stage, getting up unsteadily to his position on the count of eight.
But the red-colored shorts of Arthur soon advanced again. On the opposite side, the opponent - in rival City blue - was damaged soon, significant bruising developing under his eye as the fighters traded punishing blows.
The challenger, twenty-seven, delivered effective strikes but the champion, 34, fought fire with fire with stinging uppercuts as the intensity seldom decreased over 12 rounds.
Arthur edged the decision with official scores of 114-114, narrow lead and wider gap, but each competitor were determined and brave, all the qualities their inspiration Hatton embodied.
