UK light heavyweight Buatsi earned a win yet unimpressive debut to life under recent management with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a majority decision in a competitive ten-round contest at the boxing arena.
But Parker found success, delivering effective individual punches in a match where the techniques didn't complement each other, with frequent clinching spoiling the action.
The officials had it 96-94 in favor to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my best display but how do champions respond? They win," the victor said in the arena as he was faced disapproval from the audience.
The Londoner got back to winning after his initial pro loss - an thrilling contest with his previous opponent in last winter - to claim a 20th career victory.
Several boxing experts felt the challenger, 31, deserved the win but the British boxer experienced the second setback of his 28-bout professional journey.
The Champion Earns Victory in Dull Encounter
Buatsi's progress has stalled in recent years through contract problems and long spells of inactivity.
Previously predicted as the future of British boxing, he now finds himself trying to achieve expectations - maybe in what appears to be a last chance - under recent support.
He started carefully, with neither fighter prepared to take risks in a tentative opening few rounds.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in 2022 after breaking his hand in the contest, was making just his second fight at the higher weight class.
By the fight's midpoint, the underdog had made it competitive, causing problems for Buatsi, who utilized a tight guard to barely take the cleaner periods.
The crowd's patience wore thin because of constant clinching, with Parker going down various instances during grappling.
The underdog finished the stronger, but many rounds were hard to judge. Although, former world champion the boxing analyst - commentating on Dazn - had it strongly leaning for Parker, giving Buatsi just only one stanza.
Buatsi advances, but with the caliber of heavy hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the path to championship success - on this evidence - still appears challenging.
Ricky Hatton Homage Fuels Manchester Derby Fight
In the co-main event, Lyndon Arthur beat Bradley Rea by close verdict in a terrific Manchester vs Manchester fight to secure the European light heavyweight belt.
The arena grew hushed for a extremely emotional memorial prior to the bout as ten bells rang in memory of Manchester's own Hatton, who died 46 years old in recently.
It was the premier televised fight night in the city since the passing of Hatton. As the final chime ended, the silence gave way to a roar of "only one Ricky Hatton".
The fighting during the bout equaled the sentiment. 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 referee's count.
But the Manchester United-red equipment of the fighter soon pressed ahead again. On the opposite side, Rea - in rival City blue - was damaged quickly, significant bruising developing under his orbital area as the pair landed hard punches.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, responded in kind with powerful upward punches as the pace rarely dropped over twelve rounds.
Arthur edged the decision with judges scorecards of draw, close margin and clearer margin, but both fighters were determined and full of heart, exactly what their inspiration the legend embodied.
