British light-heavyweight Buatsi secured a victory yet unimpressive debut to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a competitive 10-round fight at the Co-op Live Arena.
But the challenger found success, delivering effective clean blows in a match where the approaches didn't complement each other, with frequent clinching spoiling the contest.
The scoring judges had it 96-94 for to Buatsi, with the third a 95-95.
"It wasn't my greatest showing but what defines champions? They prevail," Buatsi said in the arena as he was met by negative reactions from the audience.
The British fighter bounced back after his first professional defeat - an thrilling contest with his previous opponent in February - to secure a 20th career victory.
Various analysts thought the challenger, thirty-one, should have been awarded the victory but the British boxer endured the second setback of his 28-bout professional journey.
The Champion Claims Triumph in Dull Match
His career development has stalled in the past few years through promotional disputes and considerable time of inactivity.
Previously predicted as the next star of the British fight scene, he now finds himself trying to realize his potential - perhaps in what seems like a ultimate opportunity - under recent support.
He opened tentatively, with both competitors willing to take risks in a calculated opening few rounds.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in 2022 after breaking his hand mid-fight, was competing in only his second fight at the higher weight class.
By the fight's midpoint, the underdog had kept it close, causing problems for Buatsi, who utilized a tight guard to narrowly win the more effective stanzas.
The crowd's patience wore thin during constant clinching, with Parker slipping to the canvas various instances during grappling.
Parker ended stronger, but several stanzas were hard to judge. However, ex-champion Carl Frampton - commentating on the broadcast network - had it particularly wide for the challenger, awarding Buatsi just one round.
Buatsi progresses, but with the presence of power puncher David Benavidez, and Russian star Bivol dominating the weight class, the road to world glory - on this evidence - still looks steep.
Hatton Homage Motivates Local Rivalry Battle
In the co-main event, Lyndon Arthur defeated Rea by divided decision in a excellent Manchester vs Manchester battle to win the European light-heavyweight title.
The stadium became quiet for a extremely emotional homage prior to the bout as ten bells sounded in memory of Manchester's own Ricky Hatton, who succumbed 46 years old in recently.
It was the first major televised fight night in the urban center since Hatton's passing. As the last bell faded, the stillness transitioned to a shout of "there's only one Hatton".
The fighting during the bout matched the sentiment. The eventual winner was knocked down by a left hook squarely on the jaw in the second round, rising shakily to his feet on the referee's count.
But the Manchester United-red attire of Arthur soon came forward again. On the opposite side, Rea - in rival City blue - was marked up quickly, noticeable damage developing under his eye as the pair traded hard punches.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, responded in kind with powerful upward punches as the action hardly diminished over 12 rounds.
Arthur barely secured the decision with judges scorecards of draw, 115-113 and wider gap, but the two athletes were determined and courageous, all the qualities their hero Hatton represented.
