UK light heavyweight Buatsi made a winning yet underwhelming beginning to his career under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a split decision in a tight ten-round contest at the Manchester venue.
But the challenger had his moments, connecting with powerful clean blows in a fight where the techniques didn't complement each other, with excessive holding spoiling the fight.
The scoring judges scored it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my best showing but what defines champions? They prevail," the victor commented in the ringside as he was faced disapproval from the spectators.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in last winter - to claim a twentieth professional win.
Various analysts thought the challenger, 31, deserved the decision but the Derbyshire fighter suffered the second setback of his 28-fight career.
The Champion Claims Win in Dull Encounter
The fighter's advancement has stalled in recent years through contract problems and long spells of inactivity.
Once tipped as the future of UK boxing, he now stands trying to fulfil that early promise - maybe in what feels like a last chance - under fresh management.
He opened tentatively, with neither fighter prepared to overcommit in a cagey early stages.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in 2022 after suffering a hand injury mid-fight, was competing in only his second bout at the higher weight class.
By the halfway stage, the underdog had stayed in contention, making life awkward for Buatsi, who operated with a solid defense to narrowly win the cleaner periods.
The audience's tolerance wore thin during constant clinching, with the challenger falling to the floor various instances during clinches.
Parker closed well, but several stanzas were challenging to assess. Nevertheless, former world champion the boxing analyst - commentating on Dazn - had it particularly wide for the challenger, awarding Buatsi just one round.
The victor progresses, but with the caliber of hard hitter David Benavidez, and Russian star Bivol leading the category, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Memorial Inspires Manchester Derby Fight
In the secondary fight, Arthur defeated Rea by close verdict in a outstanding all-Manchester fight to secure the European light-heavyweight belt.
The arena became quiet for a extremely emotional homage before the fight as a bell ceremony rang in remembrance of the city's native Ricky Hatton, who passed away at age 46 in September.
It was the first major televised fight night in the city since Hatton's death. As the concluding sound disappeared, the silence gave way to a roar of "only one Hatton".
The fighting within the ring reflected the sentiment. The eventual winner was knocked down by a significant blow directly to the chin in the early round, standing uncertainly to his feet on the eight count.
But the red-colored attire of Arthur soon advanced again. On the opposite side, Rea - in opposing blue - was damaged quickly, noticeable damage appearing under his orbital area as the fighters traded hard punches.
Rea, 27, landed power punches but the champion, 34, fought fire with fire with stinging uppercuts as the pace hardly diminished over 12 rounds.
The victor edged the verdict with official scores of draw, narrow lead and wider gap, but the two athletes were determined and brave, everything their hero Hatton represented.
