UK light heavyweight Buatsi secured a victory yet disappointing beginning to life under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a judges' verdict in a tight 10-round contest at the boxing arena.
But Parker found success, connecting with powerful single shots in a fight where the techniques didn't complement each other, with frequent clinching spoiling the fight.
Two judges judged it 96-94 in favor to the winner, with the remaining official a 95-95.
"It wasn't my finest display but how do champions respond? They prevail," the victor stated in the ringside as he was faced negative reactions from the spectators.
The Londoner got back to winning after his initial pro loss - an epic battle with Callum Smith in February - to earn a 20th career victory.
Several boxing experts believed Parker, 31, deserved the win but the Derbyshire fighter endured the second defeat of his 28-fight professional journey.
Buatsi Earns Victory in Dull Match
His career development has stalled in recent years through management issues and long spells of lack of fights.
Earlier forecast as the rising talent of UK boxing, he now finds himself trying to achieve expectations - maybe in what seems like a ultimate opportunity - under new promotional backing.
He started carefully, with the two boxers ready to be aggressive in a cagey early stages.
The challenger, whose single earlier setback 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 middle rounds, Parker had kept it close, causing problems for Buatsi, who utilized a tight guard to barely take the better periods.
The crowd's patience decreased amid frequent holding, with Parker falling to the floor multiple occasions during clinches.
Parker finished the stronger, but multiple periods were difficult to score. However, previous titleholder the boxing analyst - working with Dazn - had it particularly wide for Parker, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the presence of heavy hitter David Benavidez, and Russian champion Bivol ruling the division, the journey to titles - on this evidence - still looks steep.
Hatton Homage Fuels Local Rivalry Battle
In the secondary fight, Arthur defeated Bradley Rea by close verdict in a terrific local rivalry fight to win the European light-heavyweight title.
The arena fell silent for a extremely emotional homage before the fight as 10 bells rang in memory of the city's native Ricky Hatton, who died 46 years old in recently.
It was the initial significant broadcast fight night in the location since the passing of Hatton. As the concluding sound ended, the quiet became a shout of "only one Hatton".
The action inside the ropes matched the sentiment. The eventual winner was sent to the canvas by a powerful punch directly to the jaw in the initial stage, rising shakily to his position on the count of eight.
But the red-colored shorts of the fighter soon advanced again. From the other corner, the opponent - in opposing blue - was damaged early, significant bruising developing under his eye as the duo traded powerful shots.
The challenger, twenty-seven, landed power punches but the champion, 34, fought fire with fire with effective uppercuts as the action hardly diminished over the full distance.
The victor edged the verdict with judges scorecards of even score, 115-113 and 115-112, but the two athletes were persistent and brave, all the qualities their inspiration Hatton embodied.
