British light heavyweight Buatsi secured a victory yet underwhelming start to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level seemingly secured him a majority decision in a tight 10-round contest at the boxing arena.
But the challenger had his moments, delivering effective individual punches in a match where the approaches didn't complement each other, with frequent clinching spoiling the fight.
The officials scored it 96-94 in favor to Buatsi, with the final judge a 95-95 draw.
"It wasn't my best performance but what defines champions? They win," Buatsi said in the arena as he was greeted with negative reactions from the spectators.
The British fighter got back to winning after his initial pro loss - an memorable fight with Callum Smith in last winter - to secure a twentieth professional win.
Some ringside observers thought Parker, thirty-one, deserved the decision but the contender experienced the second defeat of his 28-match career.
Buatsi Claims Victory in Slow Match
His career development has slowed in the past few years through management issues and considerable time of lack of fights.
Once tipped as the rising talent of UK boxing, he now is positioned trying to fulfil that early promise - maybe in what appears to be a last chance - under fresh management.
He opened tentatively, with both competitors ready to be aggressive in a cagey opening few rounds.
Parker, whose single earlier setback came at super-middleweight against John Ryder in 2022 after damaging his hand in the contest, was participating in his second bout at light heavyweight.
By the middle rounds, Parker had kept it close, making life awkward for Buatsi, who worked behind a protective stance to edge the cleaner periods.
The crowd's patience diminished because of frequent holding, with Parker falling to the floor various instances during grappling.
Parker ended stronger, but many rounds were hard to judge. Although, ex-champion Carl Frampton - providing analysis for the broadcast network - had it significantly in favor for the challenger, scoring for Buatsi just only one stanza.
The victor advances, but with the caliber of power puncher David Benavidez, and Russian star Bivol ruling the division, the path to championship success - judging by this showing - still looks steep.
Ricky Hatton Tribute Motivates All-Manchester Fight
In the co-main event, Arthur beat Bradley Rea by close verdict in a outstanding all-Manchester fight to win the European light heavyweight belt.
The arena fell silent for a extremely emotional tribute prior to the bout as a bell ceremony sounded in honor of Manchester's own Ricky Hatton, who died aged 46 in recently.
It was the initial significant broadcast match card in the city since the passing of Hatton. As the concluding sound faded, the quiet became a chant of "only one Hatton".
The action within the ring equaled the sentiment. The eventual winner was knocked down by a significant blow flush on the chin in the second round, rising shakily to his feet on the eight count.
But the Manchester United-red equipment of the fighter soon came forward again. On the opposite side, Rea - in rival City blue - was injured quickly, heavy swelling forming under his face as the fighters landed punishing blows.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, fought fire with fire with stinging uppercuts as the intensity hardly diminished over twelve rounds.
Arthur edged the result with scoring tallies of even score, 115-113 and clearer margin, but each competitor were determined and courageous, exactly what their idol the legend embodied.
