British light-heavyweight Buatsi secured a victory yet disappointing beginning to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a judges' verdict in a tight 10-round contest at the Manchester venue.
But Parker found success, landing some telling single shots in a fight where the techniques didn't complement each other, with excessive holding disrupting the action.
Two judges had it 96-94 for to Buatsi, with the final judge a even score.
"It wasn't my greatest showing but what defines champions? They win," Buatsi stated in the ring as he was faced negative reactions from the spectators.
The British fighter returned to winning ways after his initial pro loss - an epic battle with Callum Smith in last winter - to earn a 20th career victory.
Some ringside observers thought the challenger, 31, deserved the win but the contender experienced the second loss of his 28-bout professional journey.
Buatsi Earns Win in Slow Match
The fighter's advancement has stalled in recent times through contract problems and considerable time of inactivity.
Previously predicted as the rising talent of the British fight scene, he now is positioned trying to realize his potential - perhaps in what feels like a ultimate opportunity - under fresh management.
He began cautiously, with the two boxers ready to be aggressive in a cagey early stages.
Parker, whose single earlier setback came at super-middleweight against John Ryder in two years ago after breaking his hand during the bout, was participating in his second fight at light heavyweight.
By the middle rounds, Parker had made it competitive, making life awkward for Buatsi, who operated with a solid defense to barely take the cleaner stanzas.
The audience's tolerance wore thin because of constant clinching, with Parker falling to the floor multiple occasions during grappling.
The underdog finished the stronger, but multiple periods were hard to judge. Although, previous titleholder Carl Frampton - providing analysis for Dazn - had it particularly wide for the challenger, scoring for Buatsi just only one stanza.
The victor advances, but with the likes of heavy hitter David Benavidez, and Russian star Dmitry Bivol leading the category, the road to world glory - on this evidence - still looks steep.
Hatton Tribute Motivates Local Rivalry Fight
In the supporting bout, Lyndon Arthur overcame Rea by close verdict in a terrific local rivalry battle to capture the European light heavyweight title.
The stadium grew hushed for a profoundly moving tribute ahead of the contest as a bell ceremony rang in remembrance of local hero Ricky Hatton, who passed away aged 46 in September.
It was the initial significant broadcast boxing event in the city since the passing of Hatton. As the final chime disappeared, the stillness transitioned to a shout of "there's only one Hatton".
The competition within the ring equaled the emotion. The eventual winner was sent to the canvas by a powerful punch directly to the face in the early round, getting up unsteadily to his position on the count of eight.
But the red-colored shorts of the fighter soon came forward again. On the opposite side, the opponent - in rival City blue - was damaged quickly, significant bruising forming under his eye as the duo traded powerful shots.
Rea, twenty-seven, landed power punches but Arthur, 34, fought fire with fire with effective uppercuts as the action seldom decreased over the full distance.
The victor narrowly won the verdict with judges scorecards of 114-114, narrow lead and wider gap, but each competitor were relentless and full of heart, all the qualities their hero Hatton represented.
