UK light heavyweight Buatsi earned a win yet disappointing debut to his career under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a majority decision in a tight ten-round bout at the Co-op Live Arena.
But Parker landed significant shots, delivering effective clean blows in a contest where the approaches didn't complement each other, with constant grabbing disrupting the fight.
The officials judged it 96-94 for to the winner, with the final judge a even score.
"It wasn't my finest display but what do champions do? They prevail," Buatsi commented in the arena as he was met by negative reactions from the spectators.
The Londoner returned to winning ways after his first professional defeat - an epic battle with his previous opponent in February - to secure a 20th pro success.
Some ringside observers believed the challenger, 31, merited the decision but the Derbyshire fighter suffered the second defeat of his 28-bout professional journey.
The Champion Earns Win in Dull Encounter
The fighter's advancement has plateaued in the past few years through management issues and extended periods of lack of fights.
Previously predicted as the next star of UK boxing, he now finds himself trying to realize his potential - possibly in what appears to be a final roll of the dice - under new promotional backing.
He began cautiously, with neither fighter ready to be aggressive in a calculated initial rounds.
Parker, whose single earlier setback came at super-middleweight against John Ryder in two years ago after damaging his hand mid-fight, was participating in his second bout at the higher weight class.
By the middle rounds, Parker had kept it close, making life awkward for the favorite, who worked behind a tight guard to edge the more effective stanzas.
The crowd's patience diminished amid frequent holding, with the challenger going down several times during grappling.
Parker ended stronger, but many rounds were difficult to score. However, former world champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for the challenger, giving Buatsi just a single round.
Buatsi progresses, but with the likes of hard hitter David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still looks steep.
Hatton Memorial Motivates All-Manchester Fight
In the supporting bout, Arthur beat Rea by close verdict in a outstanding all-Manchester contest to win the continental belt.
The arena became quiet for a profoundly moving tribute before the fight as 10 bells tolled in remembrance of Manchester's own Ricky Hatton, who passed away aged 46 in September.
It was the initial significant broadcast boxing event in the urban center since Hatton's death. As the concluding sound ended, the quiet became a shout of "only one Hatton".
The fighting within the ring reflected the feeling. Arthur was sent to the canvas by a significant blow squarely on the jaw in the second round, getting up unsteadily to his footing on the referee's count.
But the red-colored equipment of the fighter soon advanced again. On the opposite side, the opponent - in opposing blue - was injured soon, significant bruising forming under his eye as the fighters exchanged hard punches.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, responded in kind with effective uppercuts as the pace seldom decreased over the full distance.
Arthur edged the verdict with official scores of even score, narrow lead and 115-112, but both fighters were relentless and brave, all the qualities their inspiration Hatton stood for.
