UK light-heavyweight Buatsi secured a victory yet unimpressive debut to his career under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a split decision in a tight 10-round contest at the Manchester venue.
But the challenger landed significant shots, landing some telling clean blows in a fight where the approaches never quite clicked, with frequent clinching disrupting the action.
The officials judged it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my finest performance but what defines champions? They win," Buatsi commented in the ringside as he was faced negative reactions from the audience.
The British fighter returned to winning ways after his maiden career setback - an thrilling contest with his previous opponent in last winter - to earn a 20th career victory.
Various analysts felt Parker, 31, should have been awarded the win but the Derbyshire fighter experienced the second loss of his 28-fight professional journey.
The Champion Secures Win in Slow Encounter
His career development has plateaued in the past few years through management issues and extended periods of inactivity.
Earlier forecast as the rising talent of UK boxing, he now is positioned trying to achieve expectations - possibly in what seems like a final roll of the dice - under new promotional backing.
He started carefully, with neither fighter ready to be aggressive in a cagey opening few rounds.
The challenger, whose sole prior loss came at the lower weight class against John Ryder in 2022 after damaging his hand in the contest, was making just his second bout at the higher weight class.
By the middle rounds, the underdog had kept it close, creating difficulties for Buatsi, who utilized a solid defense to edge the cleaner stanzas.
The audience's tolerance wore thin during regular grabbing, with the challenger slipping to the canvas several times during clinches.
The underdog ended stronger, but many rounds were difficult to score. Although, ex-champion the boxing analyst - providing analysis for the broadcast network - had it particularly wide for Parker, scoring for Buatsi just only one stanza.
The victor progresses, but with the caliber of hard hitter David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the journey to titles - on this evidence - still looks steep.
Ricky Hatton Homage Motivates Manchester Derby Battle
In the co-main event, Lyndon Arthur defeated Rea by close verdict in a outstanding local rivalry battle to capture the continental championship.
The arena fell silent for a deeply poignant homage ahead of the contest as a bell ceremony tolled in honor of Manchester's own Ricky Hatton, who passed away at age 46 in last month.
It was the premier televised boxing event in the urban center since Hatton's death. As the final chime disappeared, the stillness became a shout of "only one Hatton".
The competition inside the ropes reflected the emotion. The eventual winner was floored by a powerful punch directly to the chin in the early round, rising shakily to his feet on the referee's count.
But the Manchester United-red attire of the fighter soon came forward again. From the other corner, Rea - in rival City blue - was injured quickly, noticeable damage appearing under his eye as the pair exchanged punishing blows.
The challenger, twenty-seven, delivered effective strikes but the champion, 34, fought fire with fire with effective uppercuts as the action seldom decreased over 12 rounds.
The victor edged the result with scoring tallies of draw, 115-113 and 115-112, but each competitor were determined and courageous, exactly what their inspiration the legend stood for.
