UK light-heavyweight Buatsi made a winning yet underwhelming start to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a majority decision in a tight 10-round contest at the boxing arena.
But Parker landed significant shots, landing some telling clean blows in a match where the approaches never quite clicked, with excessive holding marring the action.
The officials scored it 96-94 to Buatsi, with the remaining official a even score.
"It wasn't my greatest performance but what do champions do? They win," the victor commented in the ringside as he was greeted with boos from the spectators.
The British fighter got back to winning after his initial pro loss - an memorable fight with Callum Smith in February - to earn a 20th career victory.
Some ringside observers thought Parker, 31, merited the victory but the contender suffered the second loss of his 28-match professional journey.
The Champion Claims Victory in Slow Affair
Buatsi's progress has plateaued in recent times through contract problems and considerable time of lack of fights.
Previously predicted as the future of British boxing, he now is positioned trying to achieve expectations - perhaps in what feels like a final roll of the dice - under new promotional backing.
He opened tentatively, with neither fighter ready to be aggressive in a tentative early stages.
Parker, whose sole prior loss came at super middleweight against John Ryder in two years ago after breaking his hand during the bout, was competing in only his second appearance at light-heavyweight.
By the halfway stage, Parker had stayed in contention, causing problems for Buatsi, who worked behind a protective stance to barely take the better periods.
The crowd's patience diminished because of regular grabbing, with the challenger slipping to the canvas several times during close exchanges.
Parker finished the stronger, but several stanzas were hard to judge. However, previous titleholder Carl Frampton - working with Dazn - had it strongly leaning for Parker, scoring for Buatsi just a single round.
Buatsi progresses, but with the likes of power puncher David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - based on this performance - still seems difficult.
Hatton Homage Fuels All-Manchester Contest
In the co-main event, Arthur defeated Rea by divided decision in a excellent local rivalry battle to secure the European light-heavyweight championship.
The stadium became quiet for a extremely emotional memorial prior to the bout as a bell ceremony tolled in remembrance of Manchester's own Ricky Hatton, who died at age 46 in September.
It was the first major televised boxing event in the city since Hatton's death. As the concluding sound faded, the silence became a chant of "there's just one Ricky Hatton".
The fighting within the ring matched the emotion. Arthur was sent to the canvas by a left hook directly to the face in the early round, standing uncertainly to his feet on the count of eight.
But the red-colored equipment of the fighter soon came forward again. Across the ring, Rea - in opposing blue - was injured early, significant bruising appearing under his face as the fighters traded punishing blows.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, responded in kind with effective uppercuts as the pace seldom decreased over twelve rounds.
The victor narrowly won the result with judges scorecards of even score, narrow lead and 115-112, but each competitor were persistent and brave, everything their hero Hatton stood for.
