British light-heavyweight Buatsi earned a win yet unimpressive debut to life under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a majority decision in a tight 10-round contest at the Manchester venue.
But the challenger had his moments, delivering effective individual punches in a fight where the approaches failed to mesh, with frequent clinching disrupting the fight.
The scoring judges judged it 96-94 in favor to Buatsi, with the final judge a even score.
"It wasn't my greatest showing but what defines champions? They prevail," the victor commented in the arena as he was faced boos from the crowd.
The British fighter returned to winning ways after his initial pro loss - an thrilling contest with Callum Smith in last winter - to earn a 20th career victory.
Some ringside observers felt Parker, 31, deserved the decision but the contender suffered the second setback of his 28-match professional journey.
The Champion Claims Victory in Uneventful Match
His career development has slowed in recent times through management issues and extended periods of lack of fights.
Earlier forecast as the future of the British fight scene, he now is positioned trying to fulfil that early promise - maybe in what seems like a final roll of the dice - under recent support.
He opened tentatively, with neither fighter willing to be aggressive in a tentative opening few rounds.
The challenger, whose sole prior loss came at super middleweight against his former opponent in two years ago after breaking his hand mid-fight, was competing in only his second bout at light heavyweight.
By the halfway stage, Parker had kept it close, causing problems for Buatsi, who operated with a protective stance to edge the better stanzas.
The crowd's patience wore thin amid frequent holding, with the challenger going down multiple occasions during close exchanges.
The underdog ended stronger, but several stanzas were challenging to assess. Nevertheless, previous titleholder the boxing analyst - providing analysis for Dazn - had it strongly leaning for the challenger, awarding Buatsi just one round.
The victor advances, but with the caliber of heavy hitter David Benavidez, and Russian star Bivol ruling the division, the road to world glory - based on this performance - still looks steep.
Hatton Homage Inspires Manchester Derby Fight
In the co-main event, Arthur overcame Rea by split-decision in a outstanding Manchester vs Manchester battle to secure the continental belt.
The stadium grew hushed for a extremely emotional homage prior to the bout as a bell ceremony tolled in remembrance of Manchester's own Ricky Hatton, who died aged 46 in recently.
It was the initial significant broadcast fight night in the location since the passing of Hatton. As the concluding sound disappeared, the silence became a roar of "there's only one Hatton".
The competition within the ring matched the sentiment. Arthur was floored by a powerful punch squarely on the chin in the second round, getting up unsteadily to his footing on the eight count.
But the traditional red attire of Arthur soon pressed ahead again. From the other corner, Rea - in opposing blue - was damaged quickly, noticeable damage appearing under his face as the fighters traded punishing blows.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, fought fire with fire with stinging uppercuts as the intensity rarely dropped over 12 rounds.
The victor edged the decision with judges scorecards of even score, close margin and 115-112, but the two athletes were relentless and courageous, everything their idol Hatton represented.
